The AES encryption algorithm is used by United States government departments for encrypting confidential data. It is now used for both classified and unclassified data. The algorithm has been published publicly and it is likely it will eventually be widely used in the commercial sector.
Originally the AES encryption algorithm was only designed to be used for data that was secret but not classified. Classified data is that where the law restricts which people are allowed to access the data. The AES encryption algorithm was approved as the federal government standard in 2002, then in 2003 received approval for use in classified documents. As of 2010, it was still the only publicly accessible encryption system approved for classified documents.
The AES encryption algorithm replaced the Data Encryption Standard in US government use. The Data Encryption Standard, first adopted in 1976, used a 56 bit key, meaning it could be reduced to 56 characters, each of them either a 1 or a 0. This meant that the odds of guessing the number correctly were one in 72 quadrillion, which can also be expressed as one in 72 thousand million million. Put another way, it would on average take one in 36 quadrillion attempts to correctly guess the entire key and be able to decipher encrypted messages without authorization.
At the time, this seemed so difficult it was practically impossible. As computing power increased, it became more feasible. In 1999, an experiment showed a computer could crack the algorithm in less than one day by simply trying out every possible combination of 1s and 0s. This is known as a brute force attack. Officials tried to counter this by developing "Triple DES," where there are three separate keys for each set of data, but there were still calls for a new system.
The AES algorithm is much more complicated. It consists of three separate keys, which are 128, 192, and 256 bits respectively. The odds of cracking the algorithm are, to say the least, immense. Except for people who are already familiar with the unit quattuorvigintillion, it's probably safe to say the odds are inconceivable!
That isn't to say there will never be a computer powerful enough to crack AES through brute force, but at the moment, it would be practically impossible. Even if a computer could be made powerful enough, any existing manufacturing techniques would make it impossible to actually build such a machine in a manageable size. Indeed, anyone attempting to build such a machine would probably attract the interest of the US government long before they were able to crack the algorithm.