The electronic circuit in computers detect the difference between two states: ON (the current passes through) or OFF (the current doesn’t); they represent these states as 1 or 0. Each 1 or 0 is called a binary digit or bit. Bits are grouped into eight-digit codes that typically represent characters (letters, numbers and symbols). Eight bits together are called a byte. For example, 01000001 is used for the character A. Computers use a standard code called ASCII for the binary representations of characters.
In order to avoid complex calculations of bytes, we use bigger units. A kilobyte (KB) is 1024 bytes; a megabyte (MB) is 1024 kilobytes; a gigabyte (GB) is 1024 megabytes; a terabyte (TB) is 1024 gigabytes. We use these units to describe the RAM memory, the operating capacity of disks and the size of a program or document.