Binary is base 2 number system. It is base 2 because it uses two possible numbers: 0 and 1. Decimal, the system most of the world uses for daily life, is a base 10 system – it uses 10 characters (0–9). When binary numbers are written, a subscript "(2)" is added to distinguish them from the same number in base 10.
Computers work in binary, because wires can store information in two different ways: by being powered, or not being powered. Sets of binary numbers can be used to represent any information, such as text, audio, or video.