Atrial Fibrillation is a dysrhythmia while PAC is an early depolarization coming from the atria. An occasional PAC can be normal for some people. In that case their heart rhythm would be a sinus arrhythmia instead of a normal sinus rhythm.
In AF, the atrias don’t contract/pump at all, but they quiver/twitch due to the many simultaneous ectopic foci that are being discharged from random cells; similar concept to Ventricular Fibrillation. Due to the atrias not fully depolarizing, you won’t see any P waves on the EKG. If/when an electrical discharge reaches the AV node, it goes through the normal electrical condition system from there, forming a narrow QRS complex. AF is the only irregularly irregular rhythm, meaning that the QRS complexes have no regularity at all, and have no pattern to their irregularity.
Video example of AF.
As for PACs, the atrias depolarize earlier than expected creating what looks like an early beat. The next beat though will be later than expected. The pacemaker cells “reset”, returning to their pattern after this early beat.
Both AF and PACs can be acute or chronic depending on the person.