Consider connecting one PC to another PC via a serial cable. The send terminal on 1 PC would need to go to recieve terminal on the other PC and visa versa so the cable has to be wired in a way that allows this, hence the "cross over".
When connecting to a modem, the send line is the send line on both devices and the recieve line is the recieve line, hence no cross over.
Well that's the idea.
As for practical differences, suffice it to say that you need to know which type of cable you require, they are wired differently and one can not be used in place of the other.
Jon