Regeneration is a Keyword Action.
It is most common on Green or Black creatures. It prevents a creature from being destroyed by paying the Regenerate mana cost, meaning the creature survives if it was originally going to be destroyed. Any damage dealt to the creature prior to being regenerated is removed. Any damage dealt to the creature after being regenerated remains until end of turn. There are creatures which can regenerate themselves or others and Enchantments which give a creature the Regenerate ability. There are non-Creature non-Enchantment cards which let you regenerate a creature. You need to pay (in most cases) a mana cost to regenerate a creature. Sometimes, you need to sacrifice a creature, or you may need to pay Life as well. You can regenerate a creature as many times as you want each turn for so long you can pay the cost. You have to pay the Regenerate cost each time it would be Regenerated.
When a creature CAN'T be regenerated.
- When a creature's Toughness is reduced to 0, it is sent to the graveyard as a state-based effect.
- When a creature is returned to the hand of the controller/player/...
- When a creature is already in the graveyard. You can't use regeneration to raise the creatures from death, only to prevent it.
- When a card says it can't be regenerated (example: Terror).
- When a creature is sacrificed.
A creature that has been dealt lethal damage and been regenerated gets tapped afterwards. However, creatures who are already tapped still can be regenerated, but remain tapped.
Attacking after Regeneration
When you regenerate a creature, you can attack in the same turn it was regenerated (and doesn't have Summoning Sickness or any other effect which says so) but only if you can get it untapped.
Deathtouch and Regeneration
You can regenerate from Deathtouch by paying the regeneration cost.
If a creature blocking or blocked by a creature with Deathtouch is dealt enough combat damage to destroy it, its controller does not have to pay regeneration costs twice to keep it alive. One regeneration effect is enough to prevent both lethal damage and Deathtouch.
For more on this see Magic 2010 Rules Changes: http://www.wizards.com/magic/magazine/article.aspx?x=mtg/daily/feature/42a
Trample and Regeneration
When a creature with Trample attacks and is blocked by a creature which can be regenerated, the creature can be regenerated, but regenerate does not prevent any damage done to that creature's controller from Trample. Example: You have a 1/1 creature with Regeneration and your opponent has a 4/4 Trample creature. Your opponent attacks with the 4/4 and you block with your 1/1 and regenerate. The attacking creature has to deal lethal damage before any of the damage "tramples" over. The 4/4 would assign 1 damage to the 1/1 and then 3 would trample over. Damage then resolves and the 1/1 would die, but because of the regeneration, it would not and simply be tapped instead.
What the Rulebook says
- 614.8. Regeneration is a destruction-replacement effect. The word "instead" doesn't appear on the card but is implicit in the definition of regeneration. "Regenerate [permanent]" means "The next time [permanent] would be destroyed this turn, instead remove all damage marked on it and tap it. If it's an attacking or blocking creature, remove it from combat." Abilities that trigger from damage being dealt still trigger even if the permanent regenerates. See rule 701.12.
- 701.12.. Regenerate
- 701.12a If the effect of a resolving spell or ability regenerates a permanent, it creates a replacement effect that protects the permanent the next time it would be destroyed this turn. In this case, “Regenerate [permanent]” means “The next time [permanent] would be destroyed this turn, instead remove all damage marked on it and tap it. If it’s an attacking or blocking creature, remove it from combat.”
- 701.12b If the effect of a static ability regenerates a permanent, it replaces destruction with an alternate effect each time that permanent would be destroyed. In this case, “Regenerate [permanent]” means “Instead remove all damage marked on [permanent] and tap it. If it’s an attacking or blocking creature, remove it from combat.”
- 701.12c Neither activating an ability that creates a regeneration shield nor casting a spell that creates a regeneration shield is the same as regenerating a permanent. Effects that say that a permanent can’t be regenerated don’t prevent such abilities from being activated or such spells from being cast; rather, they prevent regeneration shields from having any effect.