this was originally posted to comp.os.ms-windows.networking.misc by Steve Winograd (winograd@ibm.net). I have just copied it verbatim, since it is such a nice explanation of what a crossover cable is.
"10/
100BaseT Ethernet cards use two pairs of wires: one pair for transmitting, and one for receiving. If you connect two cards together using a straight cable, the transmit circuits are connected to each other and the receive circuits are connected to each other. It's as though you and I are trying to talk on the phone but one of us is holding the handset upside down -- neither of us will hear what the other is saying. A crossover cable reverses the transmit and receive wires between the ends:
Straight cable:
Card 1 Transmit ------ Card 2 Transmit
Card 1 Receive ------ Card 2 Receive
Crossover cable:
Card 1 Transmit ------ Card 2 Receive
Card 1 Receive ------ Card 2 Transmit
A
hub has internal circuitry which does the transmit/receive reversal."