Networking Guide : Cat 5 Wiring Scheme

Straight-through cable is a Cat 5 cable that has similar wiring in both ends. Both cable ends follow either 568A or 568B. If you buy a Cat 5 cable, a practical way to check if it is a straight-through cable is by laying the two ends (connectors) side by side and verifying the colors order. If the colors in both ends are in the same order, it is a straight-trough cable. A straight-through cable is used to connect a computer to a hub or a switch. It can also be used to connect two hubs (or switches) if each hub (or switch) has an uplink (i.e. built-in crossover) port.

Picture: Crossover Cable Wiring
568A on one end, 568B on the other end.
Pins 1, 2, 3, 6 are crossed to pins 3, 6, 1, 2 respectively.

Crossover cable is a Cat 5 cable that has one end following 568A and the other 568B. A crossover cable is used to connect two computers directly, that's without a hub or a switch. However some computer models have auto-crossover (a.k.a. auto-switching or auto-MDI/MDIX) port that lets a computer connect directly to another computer using a straight-through cable. A crossover cable is also used to connect two hubs (or switches) if both hubs (or switches) don't have uplink ports. The wiring inside a crossover cable is illustrated in the picture above.