What are "Implied Odds" in Texas Hold'em Poker?
Implied odds or implied pot odds are calculated the same way as pot odds, but consider future betting. You figure implied odds in situations where you expect to fold in the next round if you don't make your draw. You lose no more bets if you miss, but can expect to gain extra bets if you hit the draw.
An example. Say you see the flop with 2 other players acting before you in a NL game. You flop a gutshot straight draw. The first player bets and the second calls and now the pot is laying you 7:1. For a gutshot you want 10:1 or better; however, if you hit your straight on the turn, you stand to stack off one or both of your opponents. Thus your implied odds are considerably greater than 7:1, so you call.
Implied odds with a flush draw can be dicier. Namely because you can hit your flush on the turn and that kills the action. And your implied odds.
The example is a very simple one. If you were in the same scenario, but 2nd to act instead of last; you would be in a much different situation in that the last player could raise and ruin (or not) your implied odds. Additionally, stack sizes matter. If your opponents are short stacked you may not have the odds for the call. So, when considering implied odds be very aware of the situation.
While @wdypdx22's definition is correct, implied odds cannot be explained completely on a Q&A site. To fully understand the concept requires an immense amount of study and you will spend the rest of your life refining them.
A better example of when they effect the outcome of the hand more is for hidden draws. For instance, if two to a suit come up on the flop, but the turn and river give you a runner runner straight, your opponent is far less likely to put you on the hand you actually made and pay you off more.
Calculating implied odds is more of an art than a science.
Implied odds vary greatly between limit and no limit hold 'em. In either case, they occur when you are drawing to something like a straight or flush, particularly after the "turn" (fourth) card.
In limit, if you make your hand on the "river" card, and bet, you can pretty much count on each opponent to call your hand for one more (big) bet, to "keep you honest." Hence, you can mentally add these "implied" opponents' calls of your fifth card bet in calculating the size of the pot as of the fourth card.
No limit is trickier, and depends on the character of your opponents. Some (timid) NL players will call nothing after the fifth card if it looks like you made your hand. Then your implied odds after the fourth card are the same as the pot odds.
In other cases, you will have HUGE implied odds if a novice or reckless player "goes for broke" betting (or calling) that you're bluffing after the fifth card.
Most likely the result will "intermediate" to the two above. In any case, your implied odds after the fourth card depend on your ability to charge "what the market will bear" after the fifth card.
Consider calling a weak tight players preflop raise with a small pocket pair at Holdem. You only intend to continue in the hand if you flop a set. You will make a set about 1 time in 8. The amount of money you will win on average each time you flop a set needs to be at least 7 times your investment to justify calling and playing he hand this way because for every time you hit your set you will miss it seven times.
You are being laid a price to call a small amount now to win the pot plus extra action if you connect. That price (taking into account the expected future action) is your implied odds.
Most people over estimate their implied odds - you won't always get very much when you hit and hitting doesn't guarantee winning the pot.