The primary difference between the popular vote and the electoral college is that one represents the actual votes received by a candidate and the other represents the votes cast by a state. How a state casts its electoral votes is directly influenced by the popular vote.
Each state is awarded a specific amount of electoral votes based on its population. The higher the population, the more electoral votes a state receives. When the popular vote is cast, the electoral college convenes to cast all of that state's votes for the candidate who received the most votes. Because the number of electoral votes are awarded based on population, it is possible for a candidate to win the overall popular vote but not win the electoral vote. To win an election, a candidate must earn 270 electoral votes. In 2012, it was possible to win the electoral college by winning the popular vote in as few as 10 states. Electoral votes change based on population, however, so this number fluctuates from election to election. In the history of the presidential elections, there have been a handful elections in which the candidate who won the electoral vote did not win the popular vote. John Quincy Adams, Rutherford B. Hayes, Benjamin Harrison, Bill Clinton and George W. Bush won their respective elections despite a minority of the popular vote.