The Electoral College is the body which usually elects the President of the United States of America. The members of the Electoral College are called Electors. Electors are themselves elected. A faithless elector is an elector who does not vote for the candidate which they were instructed to vote for.
The Electoral College was introduced by the framers of the US Constitution to prevent a popular but unsuitable candidate from taking power. Many reformers would like to see the Electoral College scrapped in favour of a direct election, others would like to see the electoral college votes for each state allocated in proportion to the popular vote, this has already happened in some states.