The Good, the Bad, and the Ugly
Limping, as a poker strategy goes, is typically considered a pretty weak one. It happens when you call the minimum bet so that you can stay in a hand (and, in community poker, see more cards). As the Kenny Rogers song "The Gambler" teaches us, "You've got to know when to hold 'em, know when to fold 'em." Limping, more often than not, is a sign that a player really doesn't know when to fold a hand-- it indicates to the other players at the table that your hand is weak, and you just want the chance to see extra cards. While there are definitely times when it's useful as a part of poker strategy, the vast majority of the time that you hear poker commentators saying that someone was "limping into a hand," it means that the player didn't have anything worthwhile but played through anyway. Many poker coaches will tell you to avoid limping at all costs (and certainly, this strategy is a good one for new players), because there are so many things that could go wrong.
The bad: reasons to avoid limping
Rookie players think that you can limp with anything
One of the biggest mistakes that beginning poker players make is not knowing when to fold-- in essence, they limp into every hand, rather than being able to assess a hand for its playability and overall value. Not every hand can or should be limped with. What kinds of hands you want to use when you limp depends on your play style, that of the other players at the table, your position, and the reason that you're limping. Generally, limping works best when you have a hand with a high or medium strength.
You could under-play your hand
This is one of the primary criticisms of limping as a strategy. Most coaches and players will tell you that if you have a strong enough hand, you should be raising. Even if you don't have a strong enough hand, you should be raising. The general mentality among the dominant school of poker theory states that you should pretty much always either fold or raise, as limping makes you seem weak. If you miss the chance to raise and get extra money out of your opponents-- or you fail to raise and then lose the hand because you didn't weed out the other players-- then you aren't getting the maximum value that you could from each hand. Any serious poker player will tell you that maximizing your returns is the first rule of poker-- if you fail to do it, you're not playing right.
It's easy to get trapped or pot-committed
If you're limping in with a weak or middle-strength hand just to see the flop, you're investing money that the hand may not be worth. If your hand were strong enough to play, you would probably be raising or setting yourself up for a continuation bet. Many rookie players will limp in, hoping for a good draw, and by the time they realize that they're not going to get it, they've thrown good money after bad cards. Once you've limped into a hand, you may also think that it's worth it to call a small raise, in case your straight or flush makes, but doing so will only get you more pot-committed, and then you're trapped.
It sets you up as a weak player
Limping is a sign of weakness, and players who regularly limp are therefore seen as weak players. If you make a habit of limping into hands, other players are inclined to think that you either don't know when to fold or don't have a strong enough hand to raise. Either way, your opponents will feel confident that they can take your money without much of a fight.
The good: when limping might be helpful
Contrary to what beginner poker guides will tell you, there are a few good reasons to limp into a hand. While limping does give control of the hand to another player, this can be an invaluable tool in manipulating your opponents.
You want to trap an opponent
There are a couple ways to use limping to trick your opponent, but they both rely on using your knowledge of your opponent's mindset and play style. You can limp in with a strong hand and wait for your opponent to raise-- note that this works best when the other players at the table have folded-- and from there, you can either call or re-raise. Either one will surprise your opponent and throw them off their game. With a re-raise, you risk the chance of your opponent folding, which could mean that you get less than the maximum value out of the hand. You'll have to use your own judgment to determine which tactic will work best in the moment.
There are multiple callers on every hand
If you've been playing at your current table for a while and you notice that there are several players who call every time anyone raises, you might want to consider limping instead of betting. Since the entire point of raising before the flop is to scare away other players so that you have less competition for the best hand, multiple callers invalidate the value of the raise, because they're going to stick around anyway. Since you're not weeding out the competition on a hand, limping is generally the best way to see extra cards without making a raise, especially when you don't have the cards in your hand to back it up.
You're in late position
If you have a mid-strength hand-- not really good enough to bet on, but one that could do pretty decently with the right flop, you may want to limp in if you're in late position, as it will allow you to see the flop on the cheap. You risk one of the blinds raising, but if you're pretty sure that no one is going to call you (and in late position, you should have a good idea of the attitudes and style of the other players still in the hand), then limping will give you valuable information. Some strategists would advise a button raise so that you have a chance to steal the pot, but this depends on the playing strategy of the other players at the table.
You can use a limp shove to change your image
Limp shoving, or limping with a strong hand (AA-QQ) and then re-raising, works best at aggressive tables where the play is fast, and is most effective from early position. You want to use this tactic against players who think and reason through their moves, so it's best to use it in tourneys at higher-stakes tables. Your opponents perceive your limp as a sign of weakness, so they raise to knock you out of the play. When you re-raise, they're forced to either fold or call you with a hand that is weaker than yours. If your stack is big enough, you can limp shove with weaker hands. This is a great way to protect the blinds from players attempting to steal them. Either way you do it, a limp shove asserts that you're a dominant, strong player, and one that is not to be taken lightly.
The ugly: limping, in summary
While there are benefits to limping into a hand, if you have a weak hand, chances are that the flop isn't going to make it much better. If you have a strong hand, you may be losing out on the chance to make extra money-- or even your chance to win the hand. Trapping opponents is tricky, and it's not a move for the faint of heart. If you're sure that you have the basics of poker strategy down, you can try using limping as a strategy-- until then, it may be best to avoid it.