A measurement of digital information storage or transmission, and a multiple of the unit "bit" using the International System of Units prefix.
The gigabit is 10^9 bits, or 1,000,000,000 bits or 1000 megabits. A gigabit is equal to 125 megabytes.
Gigabits are pretty much only ever used to describe download and upload speeds for broadband, mobile data and dialup services as well as ethernet port speed.
For example, many ISPs will sell broadband packages of 15Mbit/s (megabits) downstream and 5Mbit/s upstream.
Therefore, to calculate how fast your actual download speeds will be, in bytes, you must divide those figures by 8. 15mbit/s = 1.875 MB (megabytes) per second.
Very similar to gibibits, which use a more exact prefix established by the International Electrotechnical Commission.