Amnesty: Allows a person to avoid prosecution. An example would be that all illegal aliens would be granted amnesty and thereby not be deported and not prosecuted for their illegal status.
Clemency: The overall legal power of the President to use his/her power to do any of the other terms listed.
Commutation: The reduction of a sentence and/or penalty. An example would be commuting a death sentence to life, or a 20 year sentence to no sentence. A commutation does not remove a conviction.
Expungement: Removing the record of arrest. Not applicable in federal cases.
Pardon: The complete removal of a conviction.
Remission: Removal of fines, costs, and/or restitution.
Respite/Reprieve: Delay of imposition of sentence.
Sealing: Preventing the public from seeing a record. Like an expungement, but the actual record still exists.