Name Equal to: Size in Bytes
Bit 1 bit 1/8
Nibble 4 bits 1/2 (rare)
Byte 8 bits 1
Kilobyte 1,024 bytes 1,024
Megabyte 1,024 kilobytes 1,048,576
Gigabyte 1,024 megabytes 1,073,741,824
Terrabyte 1,024 gigabytes 1,099,511,627,776
Petabyte 1,024 terrabytes 1,125,899,906,842,624
Exabyte 1,024 petabytes 1,152,921,504,606,846,976
Zettabyte 1,024 exabytes 1,180,591,620,717,411,303,424
Yottabyte 1,024 zettabytes 1,208,925,819,614,629,174,706,176

Then there is the hypothetical "Googolbyte" which would be a number of bytes equal to a 10 followed by 100 zeroes.

Name Example(s) of Size
Byte A single letter, like "A."
Kilobyte A 14-line e-mail. A pretty lengthy paragraph of text.
Megabyte A good sized novel. Shelley's "Frankenstein" is only about four-fifths of a megabyte.
Gigabyte The multi-player version of Diablo II, installed. About 300 MP3s. About 40 minutes of video at DVD quality (this varies, depending on maker). A CD holds about three-fourths of a gigabyte.
Terrabyte About thirty and a half weeks worth of high-quality audio. Statistically, the average person has spoken about this much by age 25.
Petabyte The amount of data available on the web in the year 2000 is thought to occupy 8 petabytes (theorized by Roy Williams).
Exabyte In a world with a population of 3 billion, all information generated anually in any form would occupy a single exabyte. Supposedly, everything ever said by everyone who is or has lived on the planet Earth would take up 5 exabytes.
Zettabyte Three hundred trillion MP3s; Two hundred billion DVDs. If every person living in the year 2000 had had a 180 gigabyte hard drive filled completely with data, all the data on all those drives would occupy 1 zettabyte.
Yottabyte ???