When would you use a crosslink cable?

Crossover cables are used when you are connecting two devices of the same type together. e.g. connecting two routers or two switches or two computers.

A straight through cable is used to connect devices of different types. e.g. a computer to a switch or router.

Using the correct cable is less important these days though as most networking devices have the capability to auto-sense what cable is being used and what cable is needed. The devices can simply change their configurations to whatever is needed to make the connections work. This technology is called Auto-MDIX (Media Dependant Interface Crossover).