G.10.1- Damage dealt to a player results in an equivalent loss of life (see Rule G.21). [CompRules 1999/04/23]
G.10.2- Damage done to a creature remains on that creature until the end of turn (actually the Cleanup Step, see Rule P.13). [CompRules 1999/04/23]
G.10.3- Damage is not removed if a permanent stops being a creature. The damage will be there if it becomes a creature again later in the same turn. [CompRules 1999/04/23]
G.10.4- If you get to choose how damage is to be distributed among multiple permanents or players, then you can only distribute whole number values, and you cannot choose zero. If a spell or ability lets you choose how much total damage is dealt, you can choose zero. [Duelist Magazine #7, Page 100, 1995/10/01]
G.10.5- Damage can only be assigned to a creature or player. If a spell or ability resolves to find that a permanent it was going to damage is no longer a creature, then it will not assign damage to it. [Bethmo 1999/12/10]
G.10.Ruling.1- Dealing zero damage to something means the same thing as not dealing damage at all. Nothing which triggers on damage being dealt will trigger on zero damage. [D'Angelo 1999/06/12]
Note- Also see Damage Prevention, Rule G.11.
Note- Also see Damage Redirection, Rule G.12.
Note- Also see Creature Power and Toughness, Rule K.10.
Note- Damage is "combat damage" only if it is done due to the combat damage part of the combat phase. See Rule C.5.
Note- A creature has "lethal damage" if it has at least as much damage on it as it has toughness. See Rule K.10.4.