If you assist someone in committing a crime or you help the offender escape arrest or apprehension for the crime then you're considered an accessory after the fact.
An accessory before the fact can be similar as well because if you assist someone in committing a crime or you encourage the person to commit the crime then you're accessory before the fact although to be an accessory before the fact it means that you weren't present when the crime was committed.
If you're present when the crime was committed then you're an accessory after the fact.
And in some States you can be charged for what the criminal was charged with even if you didn't actually commit the crime.