First moon mission – Who was the third person?

All know the fact that, NASA’s Apollo 11 spacecraft landed on moon on 20^th July 1969 and the commander Neil Armstrong became first human to walk on the moon. You might also know about the Lunar Module Pilot Adwin Aldrin Jr. who became second moonwalker. But some of you might not know that, in this mission three people went to moon, out of which 2 walked on the moon surface and third person Command Module Pilot Michael Collins orbited above the moon surface in command ship named as Columbia. So the bottom line is 3 people went to moon but 2 walked on moon surface and one was in the ship.