The 82443BX chip controls the data traffic between the microprocessor and the AGP card. Both chips share the task of controling the PCI and main memory data flow. In the diagram below, you can see the data flow paths mapped out for the chipset. Paths managed by chip 1 are red. Paths managed by chip 2 are blue.
Buses are simply the paths, or wires, that connect one component to the other.
You may have also noticed that the chips in the chipset are refered to as bridges. The chips actually are bridges that bridge the components together, and ensure that the data flow is directed to the proper place. The better chipsets are those that can handle data the fastest. Below is a picture of both chipsets on a motherboard.