The AES (Advanced Encryption Standard) is a symmetrical block-cipher algorithm with a 128-bit block size, and key sizes of 128, 192 or 256 bits. It was developed for through an international competition, and standardised by NIST in 2001.
The original candidate that was to become the AES, Rijndael, was developed by Joan Daemen and Vincent Rijmen. It is built in the form of a Substitution-Permutation framework, and utilises MDS codes to achieve rapid diffusion characteristics.
One significant reason behind its selection was that it could be implemented in a number of different ways, making it very adaptable for use on different platforms.
Some notable cryptographers were worried that the simple algebraic structures used by its internal components may have lead to a catastrophic weakness, but at the time of writing this no such flaws have been found.