A year ago the Muslim Brotherhood's candidate won the presidential election in Egypt. The contest was initially a fourway and went down to a runoff between Morsi (the eventual winner) and the Armed Forces candidate. Those opposed to the choice of Islamist or Army rule sat the election out. Morsi since then has gathered power to himself and his party, virtually freezing out the young who had led the Revolution and imposing more and more authoritarianism.
Morsi has failed to unify the country or to revive the moribund economy and today's protests are bringing together a coalition vastly larger than his just his political opponents.
Here is a link to a video from the Tamarod (Rebels) detailing what they think it is all about):