It is common to divide poker players into four types, based on two criteria: how apt are they to call (looseness vs. tightness) and how apt they are to raise (passiveness vs. aggressiveness). Note: betting when one can check is logically identical to a raise, though psychologically there is a huge difference to weak players--raising seems "aggressive," bidding doesn't. It is important to consider the type of poker players you are dealing with when making bidding decisions.

Loose-passive players

These guys (but very typically women) are also known as "call machines." They see the flop in Texas Hold 'Em no matter what. They stay in with 3-card straights. And to make things easy on you, they wouldn't raise (though they might bet!) with an ace-high flush and no pairs on the board (seriously--I see this nonsense a lot in low limit games). You don't need to change your play much if there's an extreme loose-passive at your table. You just take her money. Occassionally you'll get surprised when she beats you on the showdown (since you had no reason to believe from her bidding that she had anything). Then you'll be happy that she (or he...) saved you money.

Loose-aggressive players

Many players' worst nightmares, though on average they are worth more money than loose-passive players. Loose-aggressive players are generally compulsive gamblers, stupid, macho boys, or both. Last night I played with prototypical loose-aggressive boys. These two guys would raise every hand pre-flop, capping the bets after 3 raises half the time. One guy would put in 4 chips aggressively without looking at his cards, even though he could only raise to two. They play every hand, often hoping for miracles on the river. They bet when they should check or fold. Since they throw in so much money, loose-aggressive players raise the stakes on every hand, which really hurts loose-passive or tight-passive types who might just be trying to pass the time.

But if you are a good player, you want loose-aggressive players at your table. Good players stay in hands when, over a long period of time, they expect to win a positive X dollars/hand. If idiots randomly double every bet, then they can expect to win 2X dollars/hand. The only trick is you must be much more hesitant to play marginal hands (to see the flop or to alter your appearance to other players), because mistakes are multiplied as well. One other nice thing to note about loose-aggressive types: if they don't raise on the turn or the river, they don't have shit. If they check, they likely don't even have a pair.

Tight-passive players

If you are a good poker player, you don't want a tight player at your table. You won't win much money off of a tight player. But if he is passive as well, you won't lose much money off him. Tight-passive players tend to be males who feel they know how to play correctly--maybe they read a poker treatise that preaches the virtues of conservative play--and that aggressive players are bound to lose tons of money over the long run. If they aren't sure they are going to win even though they have a great hand, they will check or call. When these players are in you need to be more conservative about calling. If they raise, your hand needs to be very strong to stay.

Tight-aggressive players

These players are definitely the ones to fear at a poker table. I can make a strong mathematical argument that you should either raise or fold (which you should do maybe 80% of the time in a typical low-limit game) before a Texas Hold 'Em Flop (though I don't play quite so digitally--see below for explanation). If a player is throwing away most hands, but heavily betting the others, he is trouble. If he's in he has a good hand, which suggests you won't win much (or any) money off him in the long run. On the other hand he's betting and raising aggressively with his good hands, which means if you call with anything mediocre you are going to lose money over time. Additionally, a player like this, because of his tightness, has opportunity to bluff or semi-bluff (bluff with a decent possibility of winning) and cause you to fold winning hands. If you aren't a tight-aggressive player I think you're playing incorrectly.

I have seen in no other place that you should either fold or raise before the flop. Here is some (fairly superficial but I think still very reasonable and pertinent) analysis supporting that idea. Suppose there are N players who are playing before the flop and they will all stay in even after a raise (not a bad assumption for low limit hold em, and I think it's often better if people fold--though not in the case of flush draws...anyway...). Neglecting a rake, you should have about a 1/N chance of winning if you are to call, at any level of raise. Why? Say the bid is 1. Then at a 1/N probability of winning, you will lose your 1 (N-1) times for every 1 time you win the N dollars (from N players putting in 1). If the bid is 2, you will lose 2 (N-1) times for every 2 times you win the 2N dollars. Etc. For all levels of raise, you come out even if your odds of winning are 1/N. But if your odds are better than 1/N, than you come out ahead for all levels of raise, and the higher the level of raise the more you win, on average. So there is a (weak but reasonable) theoretical basis for folding any time you have a less than 1/N chance of winning and raising any time you have a better than 1/N chance of winning. I could attack my reasoning here but I think it's essentially correct. My article under raise is, I think, exactly correct and along similar lines.