What does "the nuts" mean in poker?

The best possible hand given the community cards on the board.

What is the difference between the flop, turn and river?

The flop is the first three community cards, the turn is the fourth and the river is the fifth and last.

The player directly to the left of the big blind, who acts first before the flop.

A card that is not part of your pair or other made hand, used to break ties between equal hands.

All-in: Betting every chip you have left.
Ante: A small forced bet every player puts in before a hand, used in some games and late tournament levels.
Bet: The first chips put in during a betting round.
Big blind: The larger of the two forced bets, posted by the player two seats left of the button.
Buy-in: The amount you pay or receive to start playing, or to start again after losing your chips.
Call: Matching the current bet.
Check: Passing without betting when nobody has bet yet in the round.
Fold: Giving up your hand and any chips you have already put in.
Main pot: The pot every player in the hand can win, including a player who is all-in.
Pot: All the chips bet in the current hand, won by the best hand or the last player left.
Raise: Matching the current bet and adding more.
Re-buy: Buying more chips after running out, if the game allows it.
Side pot: A separate pot for chips an all-in player could not match; only players who contributed to it can win it.
Small blind: The smaller forced bet, posted by the player directly left of the button.
Stack: The chips a player has in front of them.
Straddle: An optional, voluntary blind of twice the big blind, posted before the cards are dealt.

Board: The community cards face up in the middle of the table.
Burn card: The card discarded face down before the flop, turn and river are dealt.
Button: The marker showing who is the dealer for the hand; it moves one seat left every hand.
Community cards: Cards dealt face up that every player can use.
Flop: The first three community cards, dealt together.
Hole cards: Your private cards, dealt face down.
Kicker: A card that is not part of your made hand but breaks ties.
Muck: The pile of folded and discarded cards; also, to throw your hand away.
River: The fifth and last community card.
Showdown: When the remaining players reveal their hands to decide who wins.
Turn: The fourth community card.

Bluff: Betting with a weak hand to make better hands fold.
Draw: A hand that needs more cards to become strong, such as four cards to a flush.
Nuts: The best possible hand given the board.
Outs: The cards left in the deck that would improve your hand to a likely winner.
Pocket pair: Two hole cards of the same rank.
Set: Three of a kind made with a pocket pair and one matching board card.
Tilt: Playing badly because you are frustrated or angry.
Wheel: The lowest straight: 5-4-3-2-A.

Cutoff: The seat directly right of the button.
Early position: The first seats to act before the flop, which act with the least information.
In position: Acting after your opponent in a betting round, which is an advantage.
Under the gun: The player directly left of the big blind, who acts first before the flop.

Cash game: A game where chips stand for a fixed amount and players can join, re-buy or leave at any time.
Chop: Splitting a pot or a tournament prize pool by agreement.
No-limit: Betting rules where you may bet any amount up to your whole stack.
Tournament: A game with rising blinds where players are eliminated until one has every chip.

Playing without a chip set? With EasyPoker, the blinds, stacks and pot are on every player's phone. See how virtual poker chips work.