It is AES with key 256 bit.
AES is a standard of using the Rijndael cipher, and is the most widely-accepted encryption algorithm. It is not necessarily the most secure mathematically. The only known attacks on it right now are side-channel attacks, but that’s the fault of the implementation or platform you are encrypting on.
The Rijndael cipher was chosen because it seems to be the most performant algorithm in a variety of different systems of all bit sizes tested, and it is also extraordinarily secure. If you have control over your systems, replacing government-standard DES encryption with AES will be a great step.
Other highly-secure ciphers are Twofish, Serpent and RC6.
The following ciphers are outdated and either deprecated in favor of a newer cipher, or are a travesty of computer science: DES, Triple DES, Blowfish, and MARS.
The Advanced Encryption Standard (AES) is the algorithm trusted as the standard by the U.S. Government and numerous organizations.
Although it is extremely efficient in 128-bit form, AES also uses keys of 192 and 256 bits for heavy duty encryption purposes.
AES is largely considered impervious to all attacks, with the exception of brute force, which attempts to decipher messages using all possible combinations in the 128, 192, or 256-bit cipher. Still, security experts believe that AES will eventually be hailed the de facto standard for encrypting data in the private sector.