Switches are used to control the voltage in an electrical circuit and lamps can be used to indicate the presence or absence of voltage. Some switches are called momentary because the voltage disappears when the input "stimulus" disappears. Others are called latches because the voltage remains even after the input stimulus has gone.
By grouping a number of electrical circuits together, the presence or absence of the voltages can be used to represent numbers. We humans prefer to use numbers based on the decimal (base 10) system whilst computers work with the simpler binary (base 2) system.

A single digit in the binary system is called a bit and can only represent the deciaml numbers 0 and 1. Eight bits, when grouped together, are called a byte and can be used to represent any decimal number between 0 and 255.

The next common grouping is sixteen bits. This is called a word and can represent decimal numbers between 0 and 65,535.

After the word is the double-word which is 32 bits - the basis of all modern home computer systems (0 - 4,294,967,295).