What is a pardon?
A pardon is an act of "grace" that exempts an individual from a punishment that could be carried out by the courts. It mitigates the punishment from the law and also restores the persons rights to his /her previous standing. It restores the persons civil rights.
The power to pardon someone usually rests with the Governor of the state, or with the President of the U.S. if the crime was a federal offense.
Several different types of pardons exist such as a partial pardon, general pardon, full pardon, conditional pardon, and an absolute or unconditional pardon. Partial pardons remit only a portion of the punishment whereas an absolute pardon frees the criminal without any conditions.
From a federal Constitutional standpoint, the act of granting a pardon can be an action that the executive branch exerts that other branches cannot. The "pardon" is the executive excusing of an action. For example, a President can pardon federal offenders that they believe should receive grace. The pardon is an action that only the executive can grant, and while there might be perception associated with the actions, in the pardon the executive is able to assert what they believe is right and little can be done to prevent it. For Presidents, most pardons are issued as they are leaving office or a term of office so that there is little political fallout from such actions.