1 Bit = one binary digit (represented by a 0 or a 1).
1 Byte = 8 Bits (or one text character)
1 Kilobyte = 1,024 Bytes
1 Megabyte = 1,024 Kilobytes
1 Gigabyte = 1,024 Megabytes
1 Terabyte = 1,024 Gigabytes
1 Petabytes = 1,024 Terabytes
1 Exabytes = 1,024 Petabytes
1 Zettabytes = 1,024 Exabytes
1 Yottabyte = 1,024 Zettabytes (1,208,925,819,614,629,174,706,176 bytes or 2 to the power of 80)

For comparison:
if you downloaded a 1 YottaByte file using a 28.8 kbps modem, it would take you about 1.1 trillion years to complete download! The universe is approximately 15 Billion years old (or about 15 Giga years old).

See Wikipedia entries for more information