BNC Cables
In most cases, BNC cables are actually just another form of RCA cables but with a different physical connection. BNC's were developed around WWII for the military who were looking for a more secure locking type connection. BNC cables are commonly used these days in the professional/industrial video equipment industry. Common signal forms carried over BNC cables are:

Composite
RGB
RGBS
RGBHV

RGB Signal
A RGB signal splits the video signal into three cables: Red, Green and Blue. These cables typically carry the sync info over the green wire, sometimes calling it RGB sync on green. Common cable configurations are 3 x BNC to 3 x BNC, HD15 to 3 x BNC or HD15 to 3 x RCA.

RGBS (or RGB H/V) Signal
A RGBS or RGB H/V signal splits the video signal into four cables (Red, Green, Blue, as well as one cable for Horizontal and Vertical sync). Common cable configurations are 4 x BNC to 4 x BNC.

RGBHV Signal
A RGBHV signal splits the video signal into five cables (Red, Green, Blue, as well as Horizontal and Vertical sync). Common cable configurations are 5 x BNC to 5 x BNC, HD15 to 5 x BNC or HD15 to 5 x RCA.