Bit- The smallest unit in computing. It can have a value of 1 or 0. You'd be hard pressed to find a file size listed in bits.

Byte - A (still small) unit of information made up of 8 bits. 

Kilobyte(KB) - A unit of approximately 1000 bytes (1024 to be exact). Most download sites use kilobytes when they give file sizes.

Megabyte (MB) - A unit of approximately one million bytes (1,024 KB). 

Gigabyte (GB) - Approximately 1 billion bytes (1024 MB). Most hard drive sizes are listed in gigabytes.



OK, now for a little practical application. 

A 3 1/2" floppy drive holds 1.44 Megabytes (1,474 KB). 

A CD Rom holds 650 Megabytes (though most programs you get don't utilize the whole 650). This would be around 450 of those 3.5 floppies.

A 20 Gig hard drive will hold the same amount of info as 31 CD ROMs or 14,222 of the 3.5 floppy disks.

It takes between 4-10 minutes to download a one megabyte (1024 KB) file using the average dial up interent connection.

A typical page of text is around 4KB.

To see the size of a given file, just right-click it (in Explorer or My Computer) and select Properties from the resulting menu.