Chazara MP3 is a series of recorded shiurim covering all of Shas, Mishna Berurah, sections of Yoreh Deah, and all the Mishnayos in Shisha Sidrei Mishna. The purpose is to enable someone to review material that they're familiar with, (ie; daf yomi) quickly and smoothly. Some have even been using it to learn new Gemara. Additionally, it helps people use their time productively when traveling, etc. The magid shiur is R' Ahron Zelikovitz.