A mouse is an input device.
The terms input and output say if data flow into or out of the computer. The picture shows the major hardware components of a computer system. The arrows show the direction of data flow.
A bus is a group of wires on the main circuit board of the computer. It is a pathway for data flowing between components. Most devices are connected to the bus through a controller which coordinates the activities of the device with the bus.
The processor does the fundamental computing within the system, and directly or indirectly controls all the other components. The processor is sometimes called the "brains of a computer." The rest of the computer exists merely to supply it with information and electrical power.
In a desktop computer, the processor is about a one inch square of plastic. Inside the square is an even smaller square of silicon containing billions of tiny electrical parts. A modern processor may contain billions of transistors.
The processor is sometimes called the Central Processing Unit or CPU. A particular computer will have a particular type of processor, such as an Intel Core i7 processor or an ARM processor.
A modern processor chip (in 2014) may have up to 2.5 billion transistors.
How many transistors do you think the first commercial processor chip had in 1971? Click on a guess: