Following are major following of buses in a computer:

1. internal or local bus -- these are buses that connect internal components of the CPU, such as registers, the ALU and the control unit
2. external or system bus -- this is a bus outside the CPU that connects it to the memory and various I/O devices. There is often just one of these although mainframes and supercomputers often have 2 or more.
3. device bus -- this is a bus that connects devices to the main CPU or to each other. A SCSI bus may connect several disk drives together, for example.