The are basically two types of video conferencing.  The first is point-to-point.  The second is multipoint.

• Point-to-Point is the most basic video conferencing and most common.  It involves using individuals IP addresses to directly connect the two users.  This is the most common form for home users.  It is also used as a form of video phone.

 
• Multipoint Conferencing throws in several variations:
• The first is a reflector, multipoint connector unit (MCU) or "multipoint bridge".  This is a device which is needed for three or more users that serves as the central hub onto which all the users connect.  It can either be a stand-alone device or a computer server.
• The next is the setup of the equipment.  Point-to-Point usually uses two PCs.  In mulitipoint, however, the users can use Room Systems and Roll-About Systems in addition to PCs.
•  A room system is a large conference room which houses several cameras/microphones and allow large groups of people to participate at once.
• A roll-about system is just a cart which houses the camera, microphone and codec all in one unit.  This is good for small groups of people.
• With all of these systems, there are multiple options.  The most common are document sharing and whiteboarding.
• For a comprehensive look at all the different possible setups, check out the Video Development Initiative's site called "Video Conferencing Cookbook".