The bus allows components of the computer to communicate with each other.
Initially, the speed of the bus was 8Mhz. As we have faster CPUs, we need faster busses. The first main bus was the ISA - Industry Standard Architecture. But as CPUs got faster and needed to communicate fast with memory, they communicated with it separately (on a local bus). The bus can be whatever size that is needed. There is no longer an industry standard.
The ISA had no intelligence. The CPU decided who talks on the bus at a given time. The bus did not know who was plugged into it. So, if something was wrong, the bus didn't know the source. Now that clones have been taking away IBM's business they introduced a new bus, MCA (Multi Channel Architecture). This is a 32-bit bus with a maximum clock speed of 10 MHz and a burst speed of 40 MHz. The problem with the bus is that nothing is compatible with it and if you want to use it, you have to buy all of IBM's devices so they will be compatible. It added intelligence to the bus. Now it is bus mastered (not CPU mastered), and you can add multiple CPUs because they are not controlling the bus. The bus knows who's plugged into it. And each device has an ID number. When a device is plugged into the board, the board would recognize it and look for the driver or ask you for it (plug and play). It does its own self test at startup and if something is bad, the MCA would turn it off and allow the rest of the computer to function. The EISA (Extended ISA) took over. It is compatible with ISA. It's faster, 32-bit, 8Mhz (40 MHz burst speed). But it has limited intelligence.
Wait state: The CPU waits for memory's response for a specific amount of cycles. The more wait state (cycles), the slower it is. You can't make the bus too fast. Memory can't keep up. There's too much electromagnetism which cause interference with other electrical devices.