MP3 stands for MPEG 1 layer 3.
MPEG is an acronym for motion pictures expert group, a family of standards for displaying video and audio using lossy compression.
Lossy means that parts of the original recording are thrown out as the compressor does its business. The lower the bit-rate of the MP3 file, the more data is thrown out. This results in very small file sizes, but that lost data is gone forever. You can hear this clearly if you listen to low bit-rate MP3 files on a decent set of headphones or stereo system.
Please do not trade MP3-sourced material through etree.org. Keep your MP3s for your personal use.