Accomplice liability makes one who assists or encourages a criminal to facilitate a crime criminally liable for the crime himself.
The amount of assistance is immaterial; just some amount of assistance is required.
At common law, one could be an accessory after the fact and thereby by convicted of a crime committed by one he helped after the crime was committed. The modern approach however is to make this a separate offense with a much lower penalty.
Merely being present is usually not enough to qualify as encouragement. Rare exceptions exist however. In addition, one may be liable for the crime if he has a preexisting duty to the victim.
While causation is generally required at common law, acts that merely "raise the chance" of harm are sufficient for liability. Many jurisdictions and the Model Penal Code reject the causation requirement altogether and allow liability even when the actor's attempt to aid the principal was ineffective.
Accomplice liability's mens rea requirements are that the defendant must both have the intent to assist the primary party in the conduct that forms the basis of the offense as well as the mental state required for commission of the offense aided.