Plurality is when a candidate has the most votes, but still has fewer than 50% of the total votes. This is possible if there are three or more candidates. Majority means the candidate has more than 50%. To become president, a candidate needs at least 270 electoral votes out of 538 (majority). Each state has a certain amount of electoral votes which are not the same as actual votes. A candidate can win the popular vote by having the most individuals vote for them, but lose the election because another candidate earned the majority of the electoral votes.
Harrison was the first Whig president.