261A,
A Crossover cable simply allows an Ethernet Network Card on one computer to be connected directly to an Ethernet Network Card on another computer, rather than requiring an intermediate hub or switch.
A standard 10/100 baseT Ethernet cable is wired "Straight Through" - so that the Transmit pins at one end are connected to the Transmit pins at the other, similarly for the Receive pins.
You can verify this by holding the two ends of a cable terminated with the standard RJ45 connector side by side - the colours of the wires are identical across all 8 pins.
A Cross-over cable has a pair of the pins crossed over, such that Transmit at one end crosses over to Receive at the other. Again, by holding the RJ45 ends together, you can see that the order of the wires is not the same at both ends.
This allows two PCs to communicate directly without the necessity of an intervening switch or Hub. Obviously, this limits the number of PCs in a network to just 2.
Two PCs connected in this way can share resources (files and printers) between them - possibly also an internet connection, if one PC has a USB internet connection. Either PC can be configured as a "Server", but it is not necessary to do so to share files. As zoink says, what do you mean by Server?