A gigabyte is 2 to the 30th power, or 1,073,741,824 bytes.
It can be estimated as 10 to the 9th power, or one billion (1,000,000,000) bytes. A gigabyte is 1,024 megabytes and precedes the terabyte unit of measurement. Hard drive sizes are typically measured in gigabytes, such as a 160GB or 250GB drive. The term gigabyte is often often abbreviated as simply a "gig" in speech. For example, if you have a 250GB hard drive, you could say, "I have 250 gigs of disk space." The prefix "giga" comes from the Greek word "gigas," meaning giant.
For a list of all the different units of measurements, view this Help Center article.