Chipset is a generic term for a group of electronic microprocessors (literally meaning set of chips).
Each of those computers had a standard CPU, but much of the graphics and sound functionality was included in separate coprocessors, that worked in parallel with the main CPU.
For example, the Amiga had three coprocessors in addition to its Motorola 68000 CPU: they were named Agnus, Denise, and Paula, but to the programmer they were visible in a different way as the Blitter (graphics), the Copper (graphics and general management) and Paula (sound). Such a combination of coprocessors made those computer much more powerful than one could suspect. (see also Original chipset)
On early IBM PCs, there were no chipset of that type. But as years passed, more and more dedicated boards handled the tasks of video, sound and other peripherals, and a multi-coprocessor configuration is standard among contemporary (2003) PCs.
The term chipset is now (2003) commonly used for a PC's motherboard configuration: the usual standard is to use two microchips, the northbridge (used to "bridge" between the processor and main memory) and the southbridge (used to communicate with all the other peripherals). Different chipset technologies can have major impacts on a computer performance, especially on multiprocessor systems.