The United States Electoral College is the group of presidential electors required by the Constitution to form every four years for the sole purpose of appointing the president and vice president. Each state and the District of Columbia appoints electors pursuant to the methods described by its legislature, equal in number to its congressional delegation (representatives and senators). Federal office holders, including senators and representatives, cannot be electors. Of the current 538 electors, an absolute majority of 270 or more electoral votes is required to elect the president and vice president. If no candidate achieves an absolute majority there, a contingent election is held by the House of Representatives to elect the president and by the Senate to elect the vice president.
The states and the District of Columbia hold a statewide or districtwide popular vote on Election Day in November to choose electors based upon how they have pledged to vote for president and vice president, with some state laws proscribing faithless electors. All states except Maine and Nebraska use a party block voting, or general ticket method, to choose their electors, meaning all their electors go to one winning ticket. Maine and Nebraska choose one elector per congressional district and two electors for the ticket with the highest statewide vote. The electors meet and vote in December, and the inauguration of the president and vice president takes place in January.
Summarize what electoral college does in USA presidential election
Each states appoints electors based on methods by their legislature. 
Number of electors from each state are same as number of representatives from that state.
Candidate needs 270 electoral votes out of 538 to win presidential election.
All electors from state cast each of their vote to whoever received most votes in that state.
Maine and Nebraska can split their electoral votes based on whoever wins their district.
If no candidate gets 270 votes or higher then House of Representatives meets to elect President and Vice President.