The hexadecimal number system is the base 16 number system (as distinct from our familiar base 10 decimal number system.) It traditionally uses the digits \[\mathrm{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F},\] where \(\mathrm{A}\) through to \(\mathrm{F}\) represent the (decimal) numbers \(10\) through to \(15\). Sometimes the lower case \(\mathrm{a}\) through to \(\mathrm{f}\) are used instead.
It is commonly used in computing, as \(16=2^4\) represents \(4\) binary digits (also known as bits), so a two digit hexadecimal number represents \(8\) binary digits, which is known as a byte. Computer storage is described in bytes; for example, a 1 TB disk stores 1 terabyte, which is about \(10^{12}\) bytes.
