Computers and storage mechanisms (CD-ROMs, hard drives, USB flash drives, DVD-ROMs, etc.) need to hold much larger values than what a byte can hold (0-255). Thus, the terms kilobyte, megabyte, gigabyte, and terabyte were created to represent such large amounts of information.
The definition of a kilobyte is 1,024 bytes; however many people think of it as 1,000 bytes.
The definition of a megabyte is 1,024 kilobytes; however many people think of it as 1,000 kilobytes.
The definition of a gigabyte is 1,024 megabytes; however many people think of it as 1,000 megabytes.
The definition of a terabyte is 1,024 gigabytes; however many people think of it as 1,000 gigabytes.
Why the confusion? There are two numbering systems in play. Strictly in “computerese”, a base 2 system is in play, so that one kilobyte is 2 to the power of 10 bytes, or 1024 kilobytes. However, many hardware manufacturers, including hard drive manufacturers think of a kilobyte as only 1,000 bytes, or 10 to the power of 3 bytes.
This is the same with megabytes, gigabytes, and terabytes, and that is why when you buy a hard drive with, for example, 400 gigabytes, you may not be getting exactly the amount of room that you think!