The Queen is (usually) the person who is the consort of the person who won the Crown Tournament. Note that it is possible, and has happened, for a Queen to be the Sovereign by Right of Arms, which would make the King the consort.
The First SCA Tournament (May 1, 1966) was for the honor of naming a lady Queen of Love and Beauty. At this tournament Sir Aeginius (aka Duke Richard the Short) won the tournament, and named Marynel of Darkhaven as the first Queen of Love and Beauty. This title is sometimes used for tournaments that are not specifically Crown or other tournaments, and is not to be confused with Queen.