Trample
Keyword abilities
Excess combat damage spills over to the player or planeswalker.
How it works
When a creature with trample is blocked, its controller assigns lethal damage to the blocking creatures, then may assign the rest to the player or planeswalker it's attacking.
'Lethal' for this purpose is the blocker's toughness minus damage already marked on it. You must assign at least that much to each blocker before any damage can trample through — but you're allowed to assign more.
Common confusions
“Trample over a 0/4 wall with a 5-power attacker.”
You must assign 4 (the wall's toughness) to the wall; the remaining 1 tramples to the player.
“Trample plus deathtouch.”
With deathtouch, any 1 damage is 'lethal,' so you only need to assign 1 to each blocker — the rest can trample over. A 5/5 trample-deathtouch blocked by a 0/4 assigns 1 to it and 4 to the player.
“The blocker is removed before combat damage.”
The attacker is still 'blocked.' Without trample it deals no combat damage at all; with trample its controller may assign all of its damage to the player or planeswalker.