Draw phase: draw a card. If you have no cards, you lose.

Standby phase: nothing but some effects happen here.

Main phase 1: you may summon (play) one monster normally. Play spell and trap cards at this time.

Battle phase: your monsters can attack the enemy. If both are in attack position, it's atk vs atk. The difference is felt to the enemy in damage if victorious. If the target is in defense position, it's atk vs def. if won, no damage is dealt.

Main phase 2: if you didn't summon the first time, you can now. You can play more spells and traps.

End phase: becomes your opponents turn.