A byte is a sequence of eight binary digits. With one byte 28, i.e. 256 numbers can be written, from 0 to 255.
The byte is the computer memory unit. An alphanumeric character, for example, is coded using an eight-bit number.
The amount of memory (random access memory, mass storage, read only memory) is measured in multiples of bytes.
Originally, in order to get close to the familiar powers of ten (thousands, millions etc.), computer engineers chose the multiple 210, which is 1 024, or just over 1 000.
1 kilo-byte (kB) = 1 024 bytes (210),
1 megabyte (MB) = 1 024 kB = 1 048 576 bytes (220),
1 gigabyte (GB) = 1 024 Mo = 1.073 billion bytes (230),
1 terabyte (TB) = 1 024 Go = 1 099.512 billion bytes (240),
1 petabyte (PB) = 1 024 To = 1 259 000 billion bytes (250),
1 exabyte (EB) = 1 024 Po = 1.15 billion billion bytes (260),
1 zettabyte (ZB) = 1 024 Eo = 1 180 billion billion bytes (270),
1 yottabyte (YB) = 1 024 Zo = 1.208 million billion billion bytes (280).
For simplicity and in order to use the usual meanings of the prefixes kilo, mega, giga etc., and for marketing reasons, manufacturers of hard disks and DVD producers round off the numbers to decimal powers.
Thus a gigabyte of a DVD actually contains one billion bytes. The 4.7 GB on the label therefore represent only 4 700 000 000 bytes. The difference can be seen when you burn large files. The size of the files as displayed by a computer (e.g. in Windows) is measured in computer units, and a DVD could not hold a total of 4.7 GB (there is space taken up by the DVD formatting information).
For hard disks, the definition is variable. Their capacity is sometimes given in computer GB and sometimes in decimal GB.