First off,I'm not a Political science major.Secondly,I'm Australian.
Who wins the U.S. Presidency is decided by a system called the Electoral college voting system.Each state is allocated a number of votes which corresponds to the states House of Representatives and Senate Members.ie California has 55.New York has 31.Texas has 34.
When the vote is taken,which party has the most votes for that state assumes the whole number of Electorial College votes for that state.These votes are allocated to the 'electors' and it is assumed they will carry out the wishes of that state.All Electoral votes are counted up and the candidates who manage to get to the number of 270+(out of 538) gets the U.S. Presidency.