A blind is a forced bet made by one or two players at a poker game prior to the dealing of the cards. If a pre-deal bet is required of all players at a game and not just one or two players, it is not a blind, but is instead an ante. The house rules of some games that normally require blinds (such as Hold 'em) can require antes as well. The act of a player putting chips onto the table to satisfy the obligation to make a blind is called posting the blind.
Blinds are considered live. This means that when the initial round of betting occurs, the betting action starts after the players who posted blinds (as though the players had originally bet those amounts), but when the action circles around the table back to them, a player who posted a big blind has the option to raise the pot. This is the one time a player can seemingly "raise themeselves", but since their original bet was blind and done without the benefit of cards, this is done to provide the player in the blind with a fair opportunity to raise the pot if they desire.
When used as an adjective, blind refers to action taken prior to the deal. This is usually used in conjunction with a betting action, as when a player does a "blind call" or "raises blind". A "blind raise" is usually called a "straddle" or a "live (number)" describing both the amount of the raise and the fact that it is live (for example, in a 2/4 limit game, a blind raise must be to $4, and is often called a "live four"; in a 5/10 game, a blind raise could be called a "live ten")
The term "blind" comes from the fact that the players are being forced to bet on their hands (that is, bet as though they believed their hands were the best hands) without having seen their cards.
Generally, poker games that use blinds (such as Hold 'em or Omaha) use both a small and big blind. The small blind is the seat immediately to the left of the dealer button and the big blind is the seat immediately to the left of the small blind. In addition, anyone entering a cash game is required to either post the big blind prior to being dealt his first hand, or else wait for the big blind.
The small blind's blind amount is generally one half the size of the big blind, though that amount may vary or be rounded up or down in different casinos. In Limit hold 'em, or indeed in any poker game with a fixed-limit betting structure, the big blind is typically the same amount as the fixed limit would be on the preflop betting round (unless the pot is a kill pot). The big blind itself does count as a full bet, so in a poker room with a "one bet and three raises" cap of betting, the big blind is already considered to be the original bet part of that limit. The small blind counts as the relevant portion of a bet, so when action gets to the small blind then s long as the pot hasn't been raised he's only required to post the remaining portion to stay in the hand.
In some casinos, the player on the dealer button is also forced to post a big blind and acts as a second big blind. This "button blind" behaves in all other ways like a big blind, including the fact that it is live.
Poker games are not required to use both a small and big blind. Even games which require blinds in order to build the pot may be run using only one blind (the big blind), though having a single blind does tend to decrease the amount of betting action seen in a game, since there is only one player who has been pre-committed to the pot.
In The Blind
Sitting in a seat where you had to post a blind for the current hand is called being in the blind for that hand. The phrase is used throughout the hand, even after the deal, to indicate that you were one of the players who originally had to post a blind. Players who are in the blind typically play a much wider variety of hands, since they were already either partially or fully into the pot for a bet prior to the deal.
The phrase "in the blind" is sometimes used incorrectly when a player actually means to say they are taking an action in the dark. Since the terms are somewhat similar, it's easy to see how they could be confused.
As mentioned above house rules may require antes in addition to the blinds. When antes are used they generally range in size from perhaps as little as one twentieth up to one quarter the value of the big blind. Usually in such cases players in the blinds posts only the required blind as opposed to blind and ante combined. In tournaments that use antes as well as blinds, the size of the ante usually increases relative to the big blind as the tournament progresses.
Big Blind Special
Getting an unlikely hand in the big blind and then hitting the flop with it, oftentimes hard, is called a big blind special, and is common in low limit hold 'em. Typical big blind specials include unlikely two-pairs or straights (e.g. a player with 4-7 offsuit getting in for "free" on the big blind would be overjoyed to see a flop of 3-5-6 rainbow on the flop, since it gives them the nut straight).
Similarly, the small blind is already halfway into the pot and will often call the remaining half bet with unlikely holdings. If this position hits the flop hard, it's called getting a small blind special (of course).