Crimp on terminals are a quick and easy way to connect wires to electrical items that have terminals for connections, they can also be used to connect 2 wires together, (put more than 1 wire in a connector and you have an easy splice), there are 2 components to crimp connectors, the actual connectors and the tool used to crimp them onto the wire.

This is a small selection of crimp connectors

There are many more styles, but these are the most common ones used in automotive applications, top right is an eyelet or ring terminal, used mostly for earth connections, you put a screw through the ring, and screw it down to the body of the van, next to the ring terminal is a female blade terminal (usually called a female spade terminal, but also called a receptacle), it's the most commonly used terminal, things like relays need these terminals to mate with the male blade connections on them, under the female spade, is the male spade/blade terminal, it joins up with the female spade terminal, so can be used where you want a wire to be disconnected easily, the red ring  terminal is the same as the blue ring, but it takes a smaller wire diameter.

Other types of terminal are bullet connectors (round versions of the spade terminals, known as bullet plugs (male version), and bullet sockets (female version),
Butt splices (basically the ends of the terminals shown below, but back to back, giving a way to join 2 wires together permanently, by butting the ends together inside the connector and crimping),
Spades, different to the blade terminals mentioned above, these are more open ended versions of the ring terminals, they are designed to slide under a screw, and be tightened down, unlike the ring terminals that need the screw putting through the eyelet hole, and then screwing down.

The terminals come in different sizes, referring to the actual size of the removable part of the connector, (i.e. the blade part on a blade terminal) ring terminals have different sizes of hole in the eyelet, to suite different screw diameters, and blade terminals come in different widths to suit different currents mainly, but also for space reasons sometimes, the standard size for connections on things like relays, switches and panel mount fuse holders, is a 6.35mm wide male blade.

A lucar terminal is a particular brand of male and female blade terminal, (always 6.35mm in size), but it sometimes has a locking mechanism, consisting of a pin that is engaged into the small hole in the middle of the male blade terminal,

Above are 2 female spade terminals, shown from the place the wire is inserted, they are colour coded for different size wires, the yellow terminal takes 30 amp wire, the blue wire about 15 amp wire, red terminals take about 7.5 amp wire, you can fit smaller wire into larger terminals, but not the other way round, the yellow spade terminal here is a larger version of the blue one all over, it's for high current connections, but you can get terminals with the same size connector, but larger holes for wires (like the red and blue ring terminal mentioned earlier),
 

These are partially insulated terminals, the plastic over where the wire goes in, insulated the wire, but the actual connector part of the terminal is un insulated, these will be used in places where nothing else can touch them, not ideal for in-line applications (in the middle of wires), fully insulated terminals have a plastic covering over the whole of the metal parts of the female ones, the male terminals are always partial insulated, as they need to slot inside the female terminals.

This is what the male and female spade terminals look like in use.

To connect the terminals to the wire, you need a set of crimpers, crimpers basically squeeze the terminals onto the bared ends of wire, you could use pliers to do that, but terminal crimpers have rounded jaws, to squeeze the terminal just the right amount to make a good, strong connection between the terminal and the wire.

Above is a standard wire terminal crimping tool, this type is a multi function tool, it has wire strippers, wire cutters and bolt cutters as well as the crimping jaws, these are very cheap to buy, and easy to use, the part of the tool used to making the crimps is at the end, you can see 3 sets of half round indentations in the jaws, there are 3 coloured dots on there also, red, blue and yellow, they relate to the coloured bands on the actual crimp terminals, and you just can see the size of the half round indents are different.
The yellow terminals need a bigger hole when the tool is closed on them, to ensure it puts the same pressure all types of terminal, which is why using pliers isn't such a good idea, firstly they put flat crimps on, your not really crimping with pliers, just flattening the terminal, and secondly, the proper tool can't deform the crimp connector too much, because the holes being round, squeeze the terminal from all sides..well these ones try to do that.

To use any crimp connectors you need to first remove about 15mm of insulation from the end of the wire (here's a page on wire strippers), twist the exposed strands together as this helps when you insert the wire into the hole of the connector, if the exposed wire is too loose inside the connector, fold the exposed part of the wire over on its self, making it half the length, but double the diameter, it looks like this:-

The wire is prepared, now for the terminal, slip the terminal over the wire, and push the wire in as far as it will easily go, the terminals have a little tag sticking out as the inside end of the wire hole, this is how far you push the wire in to make a good connection.

Now you put the terminal and wire in the appropriate jaw of the crimping tool ,this case the blue hole, position the jaw of the tool so it's about 1mm from the inside end of the terminal, and squeeze hard.

The picture on the left is the result (before you squeeze, make sure the wire is still all the way inside the terminal).

That crimp has secured the terminal onto the wire electrically, now we need to put another crimp on behind the first one, to secure the rest of the terminal to the insulation, and make the connection mechanically sound.

As can be seen from the quality of the crimps made by the cheap crimping tool, they leave a lot to be desired, they do the job tho, and at around £5, you gets what you pays for.

There is another way tho, with a more expensive tool unfortunately, the ratchet crimping tool, also known as the heavy duty crimping tool, these are only crimping tools, no wire strippers or owt else, but they are very heavy duty, and if your doing a lot of crimp connections, you'll soon want one of these.

With one of these tools for making your crimps, you only need to put the terminal through it once, it makes both the electrical crimp, and the mechanical crimp in one squeeze, this is the jaw set, it actually has puts 4 crimps on every terminal at exactly the right spacing.

It's called a ratchet crimping tool because it works on a ratchet action, it wont release the terminal until you have squeezed the handles fully together, this ensures it always makes the perfect crimp, there's almost no chance you can make a weak crimp with one of these tools, they require a stronger squeeze to make the crimp, because the jaws are putting double the number of crimps on the terminal, and it is also making them a lot tighter, but you can let go of the tool mid squeeze, and it keeps the terminal in position for you to finish the crimp off, there is a release lever if you need to release it early, if you pulled the wire out before squeezing the handles for instance.

To use it, you prepare the wire the same as for the simple crimp tool, put the wire in the terminal upto the stop tag, then place the terminal in the lower jaws (they are colour coded again, the middles ones are for blue terminals), the terminal sits snugly inside the lower jaw, and you start squeezing the handles together, the jaws close quickly, then the pressure builds up around the terminal.

This is the tool 3/4 of the way through a crimp.

Once you've squeezed the handles as far as they'll go, the ratchet will release, and the jaws open up again, the terminal has been crimped with so much force that it is wedged in the lower jaws.

Remove the terminal from the jaws, and your done, the crimps done by this tool will always be the same, very strong, and uniform, it's a brilliant tool, cost's about £20, but if your doing a lot of crimping, then it's worth it in the long run, if that doesn't convince you, have a look at the difference in the quality of the crimps the 2 tools make, can you guess which one was made by this tool?

Spade terminals are probably going to be the most used terminals in a camper van conversion, if you can, try and get the fully insulated female spade receptacles, as these are as the label says, fully insulated, when the terminal is pushed home onto the matching male spade terminal, it's the exposed metal portions are covered up, so there's no need to wrap the connection in electrical tape, or use heat shrink tube (if the joint has any chance of being pulled on, then do use insulation tape, or heat shrink tube to hole the connections together)

This is a relay, showing the male spade type terminals, it needs the standard 6.35mm female spade receptacles to connect it up to a circuit.

You can use crimp terminals to splice a wire off another wire that is already connected up to something, you can actually get piggyback spade terminals, which is a female spade terminal, with a male spade terminal joined to it, the male spade sits parallel to the females spade, and you connect it to the wire your adding to the circuit, plug it into the male spade that's already in the circuit (unplugging the female spade), then plug the female spade terminal you just took out, into the male spade on the new terminal.

Or, you can splice into a circuit this way.

Simple to do, just remove the terminal from the wire that's already connected to the appliance/wire, (cut it off), strip the insulation off the old wire, do the same to the new wire, and twist them together.

Put the twisted pair of wires inside a new terminal.

And put a good crimp on the terminal, plug the terminal back in, and you now have a splice from what ever wire you attached this to, this is what you do when connecting things like lights up off a common power feed, or power outlets etc, you can usually get 2 10 amp wires in a 15 amp crimp terminal (as shown), but it's a tight fit, the twists you put on the wires to hold them together before you crimp them in the terminal can help, as the twisted strand of wire will look like a screw, you can use it to wind the wire into the terminal (you'll get so far, then the wires will start to untwist, so wind the wires in the opposite direction to tighten them up more, the wire will probably wind in backwards easier, because your tightening the twists up.

When ever you use the partially insulated connectors for mid wire joins, you will have to insulate the exposed metal portions, you can use heat shrink tube, which is best as it hold the terminals together, to un plug them you need to cut the heat shrink tube off (heatshrink tube is covered here),

Or you can use insulating tape, just wrap the terminals up in it, wind some over the ends and onto the wire's insulation too, then back up and cut the tape off over the middle of the connectors.