Fundamentally, there are three kinds of cards:

• Low cards, with which you can reliably drop the lead, even if one player is void in that suit. Generally these are the 5 and below.
• High cards, which are useful to take the tricks you want to take, and which should be dumped at the soonest opportunity if you don't want to take a trick with them. Generally these are the Queen and higher.
• Middle cards, which are the worst, because you can neither plan to lose a trick with them nor plan to win a trick with them. Middle cards are where you must use your skill to its utmost, because you must win or lose tricks by guile or else be doomed to what the other players stick on you. Obviously, these are the cards 6-Jack, and these are the cards you need to worry about having in your hand, because you never know what surprise they'll bring.