These cables are used usually when you wish to connect two similar type devices through Ethernet. They are also used for a short period of time whilst networking devices without a router or its equivalent.
Ethernet crossover cables are different from regular Ethernet cable because they have the added ability to reverse signals both ways. In fact the wires are reverse color coded, and the 1st and 3rd wires and the 2nd and 6th wires are crossed. Their packaging often distinguishes them from regular cables.
Bear in mind that these cables are suitable for direct network connection, and to use them to hookup to a hub will not produce a working link. However broadband routers are capable of sensing these cables and will let them work to connect to Ethernet equipment.