The "chipset" is, simply, what the motherboard is based on. A chipset is a combination of microchips that, together, provide a means to connect everything in a PC together, allow communication between them, and maybe throw in the odd extra feature here and there. There are Pentium 4 chipsets, Athlon 64 chipsets, and within each are several others from different manufacturers. A chipset dictates what CPU, RAM, expansion cards, drives and more a motherboard supports.
For example, a motherboard that uses the i875P chipset from Intel supports socket 478 processors (Pentium 4s and Celerons), DDR1 RAM, PCI and AGP cards.