A Premature Atrial Contraction (PAC) occurs when a focus in the atrium (not the SA node), generates an action potential before the next scheduled SA node action potential. When this occurs prior to the AV node recovering (the AV node is still refractory), a non-sinus P wave will be seen that is not followed by a QRS complex (non-conducted).
There are four main characteristics of PACs:
They are premature. That is, they occur earlier than you would expect if you were to measure the previous P-to-P intervals.
They are ectopic. Meaning they originate outside of the SA node. Thus, the P wave morphology would be different than the normal sinus P wave.
They are narrow complexes. Since they come from the atrium, they will eventually travel through the AV node and use the normal conduction system to spread to the ventricules. Unlike a premature ventricular contraction, which is wide-complexed, since it does not use the normal ventricular conduction system.
There is a compensatory pause after the PAC. The extra atrial action potential causes the SA node to become refractory to generating its next scheduled beat. Thus, it must "skip a beat" and it will resume exactly 2 P-to-P intervals after the last normal sinus beat.
When every other QRS complex is a PAC, then the rhythm is referred to as "atrial bigeminy".