An accessory after the fact is generally someone who, knowing that another person has committed a crime, helps that person avoid arrest, trial, or punishment. Historically, the crime committed by the other person had to be a felony to justify an accessory-after-the-fact conviction. However, there may be other laws available to punish those who aid misdemeanants—for example, a statute criminalizing concealment of any crime.
Example: Jane and Jesse live together in Illinois. Jesse steals one of Jane’s credit cards and charges long-distances calls to it. Jane reports Jesse to the police, and the court issues a warrant for his arrest. While the warrant is outstanding, she and he reconcile. Jane has Jesse get into the trunk of her car, then begins to drive him to his aunt’s house. Police officers stop her car and discover Jesse. They arrest him for theft of property under $300 in value, for which he is ultimately convicted. Even though she hid someone who had committed only a misdemeanor, Jane could be convicted of the felony offense of concealing a fugitive. (People v. Miller, 171 Ill. 2d 330 (1996).)
For more on this area of law, see Accomplices, Accessories, Aiders, and Abettors and Accessories After the Fact in the Boston Marathon Bombing.