The popular vote is an indication of overall numerical support while the Electoral College is a reflection of specialized voting within a constructed subset of the United States democratic process, meaning it does not necessarily reflect the will of the majority. Each state is invested with a certain number of electoral votes, which are fixed and not related to the population of said state.
Electors assigned to the Electoral College are picked or assigned based on widely differing laws based on which state assignation takes place in. The most common method of appointment is by the local branch of a given political party as most electors reflect their candidates' political views or party affiliation.
A candidates needs 270 or more of the College's 538 votes in order to cement their victory in the electoral process. Electors vote separately and the votes are counted in a special session of Congress held expressly for the purpose of tabulating those results.
Every state but Maine and Nebraska awards all electors to the victorious candidate. Maine and Nebraska divide electors based on the percentage of the vote won by a given candidate, rendering the end result more directly representative of the will of the majority.