In Pot LIMIT poker games the maximum amount players can raise or bet is equal to the size of the pot.
Fixed Limit
Fixed LIMIT poker games are games where the betting limits are structured by round.
For example a £10/£20 LIMIT Hold'em game indicates that the initial bets and raises are £10, and the later bets and raises are £20.
In fixed LIMIT games there is also a LIMIT on the number of raises per round; this can vary by poker site but is typically 3 raises per round of betting.