difference between Straight cable and Cross cable?

It's the difference in how a cable is wired. It's called the Pin-Out.

Ehternet uses 2 pairs of wires in a cable. 1 pair for Send the other for Recieve. On a standard RJ45 Jack, pins 1 and 2 are the Send Pair and pins 3 and 6 are the recieve...... a straight through cable is wired identically at both ends. a crossover cable switches the pairs 1 and 2 and 3 and 6...... the the wire connected to pin number 1 is connected to pin number 3 at the opposite end of the cable and the wire for pin 2 is connected to pin 6 at the other end..... it just crosses over the pairs.

A note about Sam's Answer. Hubs are now obsolete and all modern switches don't require crossover cables anymore because modern equiment has auto-sensing MDI/MDIX Ethernet ports.

The most basic answer is that a crossover switches the send and receive of any cable. Not just ethernet. A crossover cable it also referred to as a loopback cable. It can be used to test equipment by making a single port communicate with itself because the port on the other end of the cable is simply relaying the data directly back to the originating port (Unless it is a newer eithernet port with an Auto MDI/MDIX port... then the test doesn't work).