What's the difference between "tempo" and "control"? I hear those words tossed around a lot and am not entirely clear as to what they mean.
Tempo is about having the initiative, which allows you to dictate the attacks. Usually this relates to being able to spend all your mana, and produce as much board presence as possible for that amount of mana. In particular, cantrips are not good for tempo, since they are small for the amount of mana spent. Tempo decks tend to run things like Argent Squire so they can make effective use of early turns to start to seize control of the board. I disagree with KingZog's definition of tempo. I don't think it has anything to do with card efficiency. A tempo Rogue, for instance just runs Sprint to get back whatever card advantage is lost by playing too many cards early. Cards like Backstab and Soulfire are great tempo cards because they can remove minions for zero mana even though they cause you to spend multiple cards on your turn.
Control can mean 2 different things. "Board control" is nearly the same as "tempo", but "control decks" are completeley different. In this context, control is about limiting your opponent's ability to do what he wants. Usually this relates to playing defensively. Control decks tend to be concerned with removal and card advantage and win in the late game.