Poker Strategy
Probability is a huge factor in poker. Players use odds to determine
their actions. The chances of finishing a flush or a straight, the
probability of getting an overcard, the percentage of times you're going
to flop a set to match your pocket pair are all important factors in poker. Knowledge of these statistics is key to
winning. In online games
especially with very few (if any) tells, statistical knowledge becomes the
main factor when choosing whether to bet, call, or fold.
Here are some terms that you'll hear on this site and whenever you're
talking about poker odds...
Outs 
The number of cards left in the deck that will improve your hand.
"I had four hearts on the turn, so I had only 9 outs left to
finish that flush." 

Pot Odds 
The odds you get when analyzing the current size of the pot vs.
your next call. "There's $200 already in the pot, and only another
$10 bet coming at me, so my pot odds are good if I hit that flush." 

Bet Odds 
The odds you get as a result of evaluating the number of callers
to a raise. "With a 1 in 5 chance of hitting it, and knowing all six
of these guys are gonna call my bet, my bet odds are good too." 

Implied Odds 
The odds you are getting after the assumed result of betting for
the remainder of the hand. "Since I think these guys are going to
call on the turn and river, my implied odds are excellent." 
In poker, you commonly use outs and pot odds
the most. This is also the starting point for those who want to learn
about poker odds. To those out there who "ain't good at countin' much",
you better get good because that is how it's done. At this point it's only
simple division The numerator will be the number of outs you have. The
denominator is the number of cards left that we haven't seen. The result
will be the percentage chance of making one of those outs. Therefore, the
most math you'll be doing will be dividing small numbers by 50 (preflop),
47 (after the flop), or 46 (after the turn).
Before we move on, I must clarify one thing. A lot of you might wonder
why we never factor the opponents' cards or the burn cards when figuring
out how many cards are left. The reason is that we only consider "unseen
cards". If you saw what the burn cards were, or an opponent showed you his
hand, you would know that those cards are not going to be drawn and could
use that. We typically do not know what they have, so we don't even think
about it when talking about odds. For instance, take a standard deck of 52
cards, remove 2 Aces and burn 25 of them. If you drew the next card, what
are the chances of it being an Ace? It would be 2/50 (2 Aces left out of
50 unseen cards). It would NOT be 2/25 just because you burned half the
deck. Okay, do the same thing again, but this time you get to look at the
burn cards. Let's say that of all the cards you burned, none were an ace.
Now your odds are 2/25 because there are still 2 Aces and now only 25
"unseen cards".
By that same reasoning, let's play a game of draw poker where you get 5
cards as usual, but your opponent gets 40. Say you got Ace, King, Queen,
Jack all of Spades!, and a Four of Clubs. You get to ditch the Four and
draw one from the remaining pile of 7 cards. What are your chances of
getting that Ten of Spades? Assuming you don't get to see your opponents
hand, your chances of drawing that card would be 1 in 47 (1 Ten of Spades
in the deck, 47 "unseen cards"). It would NOT be 1 in 7. Let's say your
opponent goes to the bathroom, and you cheat and look at his hand while
he's on the crapper. If he doesn't have that Ten of Spades, that would be
1 in 7. If he did, well...it'd be 0 in 7.
Pot odds are as easy as computing outs. You compare your outs or your
chance of winning to the size of the pot. If your chance of winning is
significantly better than the ratio of the pot size to a bet, then you
have good pot odds. If it's lower, then you have bad pot odds. For example, say you are in a $5/$10 holdem game with JackTen facing one
opponent on the turn. You have an outside straight draw with a board of
259Q, and only the river card left to make it. Any 8 or any King will
finish this straight for you, so you have 8 outs (four 8's and 4 K's left
in the deck) and 46 unseen cards left. 8/46 is almost the same as a 1 in 6
chance of making it. Your sole opponent bets $10. You if you take a $10
bet you could win $200. $200/$10 is 20, so you stand to make 20x more if
you call. 1/6 higher than 1/20, so pot odds say that calling wouldn't be a
bad idea.
Another clarification...a lot of players want to somehow factor in
money they wagered on previous rounds. With the last example, you probably
had already invested a significant portion of that $200 pot. Let's say
$50. Does that mean you should play or fold because of that money you
already have in there? $50/$200? That's a big no. That's not your money
anymore! It's in a pool of money to be given to the winner. You have no
"stake" in that pot. The only stake you might have is totally mental and
has no bearing on hard statistics.
The next step is to use bet odds and implied odds. That's tougher,
because it involves predicting reactions of other players. With bet odds,
you try to factor in how many people are going to call a raise. With
implied odds, you're thinking about reactions for the rest of the game.
One last example on implied odds...
Say it's another $5/$10 holdem game and you have a four flush on the
flop. Your neighbor bets, and everyone else folds. The pot is $50 at this
point. First you figure out your chance of hitting your flush on the turn,
and it comes out to about 19.1% (about 1 in 5). You have to call this $5
bet vs. a $50 pot, so that's a 10x payout. 1/5 is higher than 1/10, so bet
odds are okay, but you must consider that this guy's going to bet into you
on the turn and river also. That's the $5 plus two more $10 bets. So now
your facing $25 more till the end of the hand. So you have to consider
your chances of hitting that flush on the turn or river, which makes it
about 35% (better than 1 in 3 now), but you have to invest $25 for a
finishing pot of $100. $100/$25 is 1 in 4. That's pretty close. But
there's more!... if you don't make it on the turn, it'll change your outs
and odds! You'll have a 19.6% chance of hitting the flush (little worse
than 1 in 5), but a $20 investment for a finishing pot of $100! $100/$20
is 1 in 5. So the chances would take a nasty turn if you didn't hit it!
What's makes it more complicated is that if you did hit it on the turn,
you could raise him back, and get an extra $20 or maybe even $40 in the
pot.
I'll let it go at that, as once you've mastered simple outs and pot
odds, bet and implied odds are just a longer extension of these equations.
If you sit and think about these things while you play, it'll come to you
eventually without any tutoring.