A rollover cable is used to join a PC (or dumb terminal) to a Cisco router. Basically it looks just like your average piece of Cat 5 with an RJ-45 end piece, but inside it's a little different.
As the name suggests one end of the cable is the reverse of the other. Although you can make the cable with any sequence you like, generally you would use:
Orange -> Brown/White
Green -> Blue/White
Blue -> Green/White
Brown -> Orange/White
Orange/White -> Brown
Green/White -> Blue
Blue/White -> Green
Brown/White -> Orange
That's if you were building the cable yourself. However most of the cables that you buy are flat and blue with wires of unusual colours. These are usually extortionate compared to the price of Cat 5, but generally big businesses buy them anyway.
The cable goes into a console port on the router, on the other end the PC (or dumb terminal) must have a DB-25 or DB-9 adapter to plug into its serial port. The connection is setup as 8 data bits, no parity and 2 stop bits in your favourite terminal emulator (eg HyperTerminal or TeraTerm in Windows) then just hit enter a few times and away you go.