Computer Buses are internal cables that are made up of a number of lines (wires), that are used to transmit data between the different hardware components on and around a motherboard (in the case of internal buses).
Data through buses is transmitted in the form of a binary 'phrase' or 'word' of a certain length (e.g 110110 has a phrase length of 6 digits). Each binary bit in a 'phrase' is transmitted through a single line inside the bus, and all of the bits travel in synchronisation along the bus as a whole.
The number of lines in a bus defines the number of digits that can be transmitted per phrase - thefore, a bus with 64 lines can transmit a phrase with a length of 64 bits (hence this is one of the differences between 32 and 64 bit systems).
There are 3 buses that can be found…