Sojid,
These are basic signaling standards for T1/E1. There are really 2 basic types. ISDN (such as PRI signaling) and CAS. ISDN signaling uses one of the channels on the T1 or E1 to signal call setup/tear-down messages along with other information like ANI, etc. So with a T1 (normally 24 channels), channel 24 is used for this signaling information. This channel is called the D-channel. On an E1 channel 16 is used as the D-channel. The remaining channels are then used for the actual voice. The benefit of ISDN is the connection time is very fast and carriers can offer many services on top of ISDN.
CAS (Channel Associated Signaling) is when a portiion of each channel of a T1 or E1 is used to signal the call connection/teardown so instead of 64kb available for voice you really only have 56kb. This generally does not cause a problem with voice communications. The main down side to CAS is that it is much slower than ISDN.
If you need more in-depth information, I suggest you research ISDN and CAS on the web.
Mike