AES (Advanced Encryption Standard) is a symmetric cipher defined in Federal Information Processing (FIPS) Standard Number 197 in 2001 as the federal government approved encryption algorithm.
The NSA has approved 128-bit AES for use up to SECRET level and 192-bit AES for use up to TOP SECRET level.
AES is based upon the Rijndael algorithm, which was invented by Joan Daemen and Vincent Rijmen. AES specifies three approved key lengths: 128-bits, 192-bits and 256-bits.
Nicolas Courtois maintains an excellent web page on the current state of attacks on AES.