What is the difference between a crossover cable and a straight through cable?

A crossover cable is used to connect two computers, two hubs , two routers ect...

A straight through cable is used to connect a computer to a router a switch or hub.

In the crossover cable the wires linking the pins are crisscrossed, while in the straight through cable each
pin connects to the same pin on the other side.

it is not possible two link two computers with only a straight through cable. 

The image above shows how the cable are inserted into the RJ45 connector. On a straight through both ends are the same.  

On a crossover cable one end of the cable will have green and orange crossed over (see image above).