A megabit is 1 million bits, or 1/8th of a megabyte. Although files are usually measured in megabytes, internet speed is usually advertised in megabits to make it seem faster.
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.