Basic poker rules..

Call: A bet exactly equal to the highest bet made by the preceding player to stay in the play. Call is often known as "stay in".

Fold: A player discards his hand and gives up his chance of winning the pot in this play. A player who folds is considered inactive and also forfeits his previous bets. Fold is also referred to as "drop out".

Raise: A bet equal to the highest bet made by any of the preceding players plus some extra amount. The additional amount is that by which he raises the bet. Every other player in the game must either then call (bring his bet up to the raised amount), fold, or raise again.

Check: It's like pass, where a player doesn't bet any amount and still stays in the play.

Also see how limit, no limit and pot limit poker rules affect your betting.