A gigabyte is 1 billion bytes. A typical standard definition movie is about 4 gigabytes in size. A related unit, the gibibyte, is 230 or 1,073,741,824 bytes.
A megabyte is 1 million bytes. A typical MP3 song is about 4 megabytes in size. A related unit, the mebibyte, is 220 or 1,048,576 bytes.