Gb means GigaByte
A gigabyte is a unit of information and is derived from Giga which means 1 billion. In that case, 1GB is equal to 1 billion bytes. However, there are people who find that 1 gigabyte 1024 * 1024 * 1024 (= 1,073,741,824) bytes should be. To denote this difference, GiB is also used for this latest version (Gibibyte). If you buy a computer or a loose hard drive in the store from 100 gigabytes, it would be possible for the installation to show that only 93 gigabytes are available.