memory.

bit = 0 or 1 (one digit)
8 bits --> e.g. 00000000 --> 1 byte
....................10101010

1000 bytes = 1 kilobyte
1000 kbyte = 1megabyte
1000 mbyte = 1 gigabyte
1000 gbyte = 1 terabyte

e.g. ASCi code
"a" --> 01100001
"b" --> 01100010