MP3 is the name commonly given to the audio formats specified by MPEG-1 Layer III and MPEG-2 Layer III, standardized as ISO/IEC 11172-3:1993, with some additions in ISO/IEC 13818-3:1995. It uses lossy compressed data.
MP3 is based in part on work by the Fraunhofer Institute, which held patents in the format. Other companies may also have held patents encumbering its implementation. One of the relevant patents was U.S. Patent 5,812,672, which expired in September 2015; many other related patents in various countries expired earlier. The final expiration of all relevant patents took until 2017, however, due to some "submarine patents" which were kept in the application process for years, extending their expiration dates. A modified version of DCT compression (Discrete cosine transform) is used, a lossy compression method also used in JPEG images.
ID3 tags are often used to provide metadata in MP3 files, though they aren't part of the MP3 specification.