Pot limit

A game played with a pot-limit betting structure allows any player to raise up to an amount equal to the size of the whole pot before the raise.

For example, let us assume that there is $10 in the pot at the start of a betting round. The first player may open the betting for up to $10. If he does in fact open for $10, the next player may raise to $40 (after calling the $10 bet, the total amount of the pot is $30, so he may raise $30). The third player would be entitled to raise to $140 (after calling $40, the pot would contain $100, thus he may raise $100). Any player may also raise less than the maximum so long as the amount of the raise is equal to or greater than any previous bet or raise in the same betting round.
Some pot-limit games make exceptions to the method described above when calculating the maximum raise in the betting round before the flop:

Some structures treat the small blind as if it were the same size of the big blind in computing pot size. In such a structure, a player can open for a maximum of four times the size of the big blind. For example, if the blinds are $5 and $10, a player may open with a raise to $40. (The range of options is to either open with a call of $10, or raise in increments of five dollars to any amount from $20 to $40.) Subsequent players also treat the $5 as if it were $10 in computing the pot size, until the big blind is through acting on the first betting round.
If the action folds all the way around to the small blind, the maximum amount the small blind can raise is also not universally agreed upon. Some games treat the big blind as a "raise" of the small blind for the purpose of calculating the maximum raise—the small blind is allowed to call the big blind, and then make a pot sized raise of twice the big blind, for a total bet of three times the big blind. Other games treat the blinds as dead money for the purpose of calculating the raise, and allow the small blind to make the same size raise as any other player, i.e. a total bet of three times the big blind plus the small blind.
Because of the disparity in methods of calculation, and the fact that the issue is certain to come up often, most major tournaments will announce the amount of the maximum opening raise to all players any time the betting limits are increased.