In Pot LIMIT poker games the maximum amount players can raise or bet is equal to the size of the pot.
No-limit
In No LIMIT poker games, there is simply no structure or maximum LIMIT on the bet or raise (except to the extent that you are limited to betting the amount of chips you have on the table).
In no LIMIT games there's usually a maximum number of chips you can buy in with. This varies depending on the sizes of the blinds.
E.g. For a £1/£2 game the maximum number of chips you can start with is £200 this is to prevent people buying a strong chip lead in the game.
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.
