The Advanced Encryption Standard (AES) specifies a FIPS-approved cryptographic algorithm that can be used to protect electronic data.
See FIPS 197 for more information.
AES encrypts and decrypts data in 128-bit blocks, using 128-, 192- or 256-bit keys. The nomenclature for AES for the different key sizes is AES-x, where x is the key size (e.g., AES-256).
See NIST SP 800-57 Part 1 Rev. 4 for more information.