Wire crimpers are tools that most network administrators will find themselves using at some point. Basically, a wire crimper is a tool that you use to attach media connectors to the ends of cables. For instance, you use one type of wire crimper to attach RJ-45 connectors on Unshielded Twisted Pair (UTP) cable, and you use a different type of wire crimper to attach Bayonet Neill Concelman (BNCs) to coaxial cabling.
In a sense, you can think of a wire crimper as a pair of special pliers. You insert the cable and connector separately into the crimper, making sure that the wires in the cable align with the appropriate connectors. Then, by squeezing the crimper's handles, you force metal connectors through the wires of the cable, making the connection between the wire and the connector.
If you do need to make your own cables instead of buying them, it is a good idea to test them before putting them on the network. It only takes a momentary lapse to make a mistake when creating a cable, and you can waste time later trying to isolate a problem in a faulty cable. If you do choose to make your own cables, you should test them with an electronic 'patch tester' before installing them on your network.