The different number you will, undoubtedly, see is because some will use the International Standard (ISO) number which is 1 GB = 1,000 MB; 1 MB = 1,000 KB; 1 KB = 1,000 Bytes. In computer technology, however, figures are dealt with in binary code. To differentiate the two, the International Standard now states that 1 GB = 1,024 should be pronounced Gibibyte, and written as GiB, rather than Gigabyte (GB) which, they say, is the correct term for being in multiples of 1,000. (They also state that KB (Kilobyte) is in multiples of 1,000 and that the correct computer term should now be pronounced as Kibibyte (KiB).)
In short, a bit is one digit, that can only be 1 or 0. A byte is 8 bits (2 to the 3rd). A kB (Kilobyte) is 1024 bytes. Megabyte is 1024 kB or 1048576 bytes, and gigabyte is 1024 megabytes. Since the bit only has 2 values, the 1024 is a power of two. Normal metric units are based on powers of 10, so kilo- mega- giga- are all nice powers of ten. -- For a long answer, see http://en.wikipedia.org/wiki/Bit or http://en.wikipedia.org/wiki/Data_rate_units