The chipset is hardware that is hardwired into your mobo and require drivers to enable it's features fully.
You know how there are several manufacturers that have a GeForce 4 card available? These manufactuers are using the GeForce 4 chip and build their graphics cards around that chip.
Basically that is how motherboards are made as well. There are really only 4 main Chipset Manufacturers currently: Intel, SiS, NVidia and VIA. That's it. Each manufacturer has a couple of current chipsets available and mobo manufacturers use those chipsets. Boards made by different manufacturers utilizing the same chipset will basically have very similar features.
A chipset consists of a Nortbridge and Southbridge. The North is the controller for dataflow between the CPU, Memory and AGP. This is a very important chip. The South is responsible for everything else like IDE, PCI, USB, etc...traffic.
These two chips function on a basic level with the default Windows drivers, but require specific drivers from their respective manufacturer to fully enable all of their features...much like a video card has basic functions with default Windows drivers, but you really need it's own drivers to enable more features.
Here is a block diagram of the SiS648 chipset:
These are the list of features that will be available to any mobo that uses this particular chipset.
The very best way I find to stay current on what features are available for mobos is to stay current on the available chipsets...then look at mobos with that chipset. Makes understanding the differences between mobos sooo easy.
This is why there is no such thing as a P4 RDRAM mobo that supports 8x AGP...there is no such chipset as of yet...end of story...no need to go searching all of the manufactuers for this kind of board...get it?
Probably more info than you were looking for, but I hope it helps.