Cable clamps are used in
theatrical riggings. It's a U-shaped bolt with a saddle and two
nuts. They generally look a little like a
padlock with a much smaller bottom and two
bolts sticking out of the base, but they can be much flatter. (Photos below).
You put a cable through the U-shaped part and attach the end with the bolts to something else. Sometimes this may be a curtain you want to hold up, sometimes it may be a sturdy wall with which you want to hold the cable up. The basic idea is that they provide a way to attach things to cables and cables to things. Usually at least two are used together.
As near as I can tell, a Crosby Clamp is a brand name of cable cramps invented by Oliver Crosby
Cable clamps are also clamps in which you can hold your reels of cable.
rootbeer277 says 'these are also used to clamp things to electrical conduit and water or gas pipes'.
Photographs
If these URIs become broken sometime in the future, please let me know::
http://wolfadventures.org/Ropes%20Course%20Standards/images/02.jpg
A 'flat' cable clamp: http://www.excesssolutions.com/mas_assets/thumb/ES1765.jpg
The reel-of-cable type: http://www.rockler.com/rockler/images/12597.jpg