USA Election problem?

Visit the yahoo home page a href="http://www.yahoo.com"http://www.yahoo.com/a in the home page you can see " Latest Election Polls and Coverage " just above the yahoo news links.It shows Obama and maccain s votes .Above the red line of the maccaain votes you can see a line "Projected Electoral Votes (270 needed)" My question is what is the meaning of "270 needed' ? who need 270 votes? Why ? who gives ths votes to maccain or obama?