Think of it this way:
A CPU does one things great: take information that it's given, and process that information to give results. However, it needs a way to get the information to process, and to output that information to a user. Information can come from many sources: Hard drives, keyboards, ethernet connections, mice, etc, and can be outputted to just as many sources (plus your monitor).
With all these sources in input and output (I/O), there needs to be some way to managed them. Putting it on the CPU is a possibility, but what that ends up doing is making them more complex, and as a consequence, less able to do exactly what they were meant to do (compute information as fast as possible). Thus, the management of the I/O devices pushed down a level to the chipset (Intel uses a dual chipset (North and south-bridges, to handle different I/O sources separately).
Different chipsets provide different I/O device options, and offer different levels of performance between the same I/O devices. For example, some chipsets provide a PCIe interface for graphics cards, while others provide an AGP interface. Similarly, this expands to it's SATA capabilities, USB, PCI, network connections, firewire, etc.
On top of this, motherboard manufacturers will decide which features of a chipset they want to use, which they don't, and how to expand beyond what a chipset provides itself (i.e. the nF4 chipset provides 4 SATAII on it, but some high end mobos like the A8B premium add additional SATA ports by bridging a second SATA controller onto the excess PCIe lanes provided by the chipset that aren't used by the PCIe graphics interface).
If you want to know more about them, it's best just googling for information, or even better is to read reviews about chipsets and motherboards. It'll give you a more practical understanding of what they do, and when you come across something you don't understand, research that part.
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