Classes 3-7
The suited hands and small pairs.
As a rule, these hands (e.g., the nine of clubs and the ten of clubs, or the ace of hearts and the six of hearts) do better in situations where many players are contesting the pot. This is because much of the money you will win from these hands comes from their ability to make straights, flushes, and sets (three-of-a-kind) which are strong hands and usually good enough to win the pot. However, since these hands don't happen all that often, you'd like as many players as possible in the pot, to ensure that your 'pay off' is as high as possible those times you do win the pot.

RULE OF THUMB: Here is the best way to decide whther to play a class 3-7 hand : take the number of the class in which the hand has been listed and subtract two. If the number of players who have already called ( not raised) is equal to or greater than the number you come up with, you can profitably play the hand.  For example, A9 is a 'class 5' hand. If you subtract two from five, you get three. Therefore, you need at least three players to have already called in order for you to play.

A9 = class 5 , so: 5 - 2 = 3 , therefore: you need at least 3 callers before you in order to call.

An important factor in your decision to play a class 3-7 hand is where you are seated at the table in relation to where the dealer is seated during that hand. This is considered your "position". You may have noticed that if you happen to be in early position, you cannot call as often as you can if you are in late position.  Using the above example, you cannot correctly play A9 if you are in first, second, or third position because it would be impossible for there to have been three callers before it is your turn.  Your relative position is an important aspect of the game.  When you are in early position, you don't have the advantage of seeing how well other players like their hands before it is your turn.  You also don't know how many other players will be continuing with their hands and whether anyone will raise. When you are in late position you are able to make more informed decisions about whether to bet, raise, or fold based on what the players before you have done. 

The exception to the 'rule of thumb'  occurs with KQ and AJ, where the two cards are not of the same suit. With these hands you can call as long as you are in middle or late position . If everyone has folded to you, and you have one of these two hands, strongly consider raising with them. You can still just call if you're only four or five players to the left of the big blind, but as you get closer to the button a raise become more necessary IF nobody else has yet entered the pot. This is because you'd like to limit the number of players that are going to enter the pot, which is an objective you can frequently accomplish by raising. These two particular hands do better in situations where there are few players contesting the pot. If someone calls after you it's still not a disaster, since you'll flop 'top pair' (example: you have KQ and the flop comes QT7, or KJ5) about 1/3 of the time, and usually this will be the best hand on the flop.

Class 8: The big, unsuited hands.
These are still okay hands, but most players lose way too much money with them. I don't put a lot of stock in these hands (although I'll still play them when the conditions are right), since it's very easy to end up with a second-best hand (for a more detailed treatment on this subject, read Hold em for Advanced Players). I only recommend playing these hands either on the button or one position to the right of the button. Also, you'd like to have few callers already in the pot when you play these hands, since it's then more likely you'll win the pot when you only flop top pair. There will be times when you'll be sorely tempted to play these hands in early and middle position, since you'll usually see your opponents doing just that-and often dragging big pots with them in the process. When you feel this urge, fight it off! These hands are trouble. Remember earlier, when I discussed how A T plays against A J? Well, the same considerations apply here.

Classes 1-2: Big pocket pairs, and big suited hands.
Ah, yes. Just like Vanessa Williams, I 'went and saved the best for last'. In general, these are your 'money makers' in hold 'em. With hands like pocket aces (AA) and pocket kings (KK), along with ace-king suited, you'll typically want to raise and reraise before the flop in an effort to get as much money in the middle as possible. The other hands listed here are also premium hands, although there are spots with these when you'd just rather call before the flop as opposed to raising, or just call a raise as opposed to reraising.

These two classes often play the same pre-flop, in that you'll often be raising with hands from both classes IF nobody else has raised in front of you. But, there are some subtle differences in the way these hands are played pre-flop.

Situation 1) Two or fewer callers in front of you.
I raise with all the hands in both classes in this spot. You can consider just calling with JJ, TT and 99 if the game is quite 'loose', meaning you'd expect at least five players to call you if you raised.

Situation 2) Three or more callers in front of you.
Just call with JJ, TT, and raise with the rest of the hands.

Situation 3) Someone has already raised.
This spot is tricky. IF I think less than four players will call me, I'll reraise with pocket Jacks through Aces and AK, and call with the rest of the hands. If five or more people are going to see the flop no matter what, I'll reraise with all the same hands except for JJ. I'll also just call with AKo if the original raise came from a reasonable player in early position.

Situation 4) Someone has raised, and someone else has reraised. 
I fold with everything except AA, KK, AK suited , QQ if the three-bettor (the re-raiser) seems like a reasonable player. If you hold AA or KK you need to four-bet (re-raise again). With AK suited and QQ you can just call. The exception to this is when someone has 'open-raised' (i.e., a player has raised and nobody who acted before him had either raised or called) on the button, or one to the right of the button, and the player in the small blind has reraised. At this point I'll call in the big blind with the J's, the AK and the AQs. If I think that by reraising again (otherwise known as 'four betting') I can get the button to fold, I'll four-bet.

In some spots, these rules may seem counterintuitive; for example, you may be wondering why I'd just call with JJ in situation #2, and raise with AQ suited. After all, isn't JJ a stronger hand? Well, yes and no. If we were to play JJ vs. AQ suited heads-up(one-on-one), with no future betting, then the J's are a slight favorite. However, you have to consider how these hands play . With J's, T's, and 9's it's very easy for a one or more cards to flop that are higher than your pair; when this happens, it becomes difficult to play your hand well. Notice that if you have TT, and the flop comes A 9 6, you only have two wins (one of the two remaining Tens in the deck) against someone holding an ace. If someone bets you'll often have to fold, since you don't have much of a chance of improving to the best hand AND you can't be at all sure that you have the best hand now. With a hand like AQ suited, you'll 'flop good' about 40% of the time; roughly 10% of the time you'll flop a flush draw (two cards on the flop that are the same suit as your hand), and about 30% of the time you'll flop either an A or a Q. Hence, 40% of the time you'll get a flop that allows you to profitably bet, raise and even reraise.