ThinNet is a name often given to thin flexible Coax cable used for old style 10Base2 networking. It is also called Ethernet II cable. It has BNC style connectors at each end and requires T connectors and terminators to attach to PC's.
It's called ThinNet because the original style Ethernet was much Thicker.
LindsayDruett says, someone needs to spend some time with him pulling wire...
ThinNet is around 4-5mm in diameter (same diameter as Cat 5), the proper thinnet cable is coloured grey, however, people have used 50 ohm coax which is normally used for radio equipment installations and is coloured black.
ThickNet is around 8-10mm in diameter, I've only seen the proper cable used which is coloured yellow.
Don't forget that each end of the cable is terminated with a 50 ohm terminating resistor, and ONE END is earthed.