A maiden over is an over in which no runs are scored that count against the bowler (so leg byes and byes may be scored as they are not counted against the bowler). A wicket maiden is a maiden over in which a wicket is also taken. Similarly, double and triple wicket maidens are when two and three wickets are taken in a maiden over.
After six deliveries the umpire calls 'over'; the fielding team switches ends, and a different bowler is selected to bowl from the opposite end. The captain of the fielding team decides which bowler will bowl any given over, and no bowler may bowl two overs in succession.