MP3 (or mp3) as a file format commonly designates files containing an elementary stream of MPEG-1 audio and video encoded data, without other complexities of the MP3 standard.
In the aspects of MP3 pertaining to audio compression the aspect of the standard most apparent to end users (and for which is it best known) MP3 uses data compression to encode data using inexact approximations and the partial discarding of data. This allows a large reduction in file size when compared to uncompressed audio. The combination of small size and acceptable fidelity lead to a boom in the distribution of music over the Internet in the mid to late 1990s, as an enabling technology when bandwidth and storage were still at a premium. The MP3 format soon became associated with controversies surrounding copyright infringement, music piracy, the file ripping/sharing services. With the advent of portable media players, a product category also including smartphones, MP3 support remains near-universal. by Wikipedia