locking means no-one can add further posts, sticking means that the forum stays at the top of whatever list of forums you are looking at.
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A locked and stickied (stuck?) topic can be updated by admins.
For example in the chess community forum you will find http://www.chess.com/forum/view/community/chesscom---list-of-caught-cheaters
What is the difference between locking and sticking a forum?