Rules
Checkmate wins. Stalemate draws. The difference is one of the most important — and most misunderstood — points in chess.
Checkmate is when the king is in check and has no legal move to escape — it can't move to safety, the attacker can't be captured, and the check can't be blocked. The game ends and the side delivering mate wins.
Stalemate is when the player to move has no legal move but their king is not in check. The game ends instantly in a draw. The only difference from checkmate is whether the king is currently under attack — yet the results couldn't be more different.
Stalemate is how losing players save games. If you're winning easily and carelessly leave the enemy king with no move while it's not in check, you throw away the win. When ahead, always give the enemy king a safe square until you're ready to deliver mate.
Besides stalemate, a game is drawn by: insufficient material (e.g. K vs K), threefold repetition, the 50-move rule (no pawn move or capture in 50 moves each), and mutual agreement. Perpetual check forces a draw through repetition.
Check the king's status before you trap it: in check with no escape = you win; not in check with no move = you only draw. Practice converting won games and watch out for stalemate tricks.
In checkmate the king is in check with no legal move, and the attacker wins. In stalemate the player has no legal move but their king is NOT in check, and the game is a draw. The king's check status is the only difference.
A draw. Many beginners lose half a point this way by trapping the enemy king without giving check. When you're winning, always leave the enemy king a legal move until you can deliver actual checkmate.