So you get extra atrial beats, from somewhere in the atria outside the SA node. The P waves therefore look odd, esp if they fall on top of a T wave. They can even be upside down if close to the AV node, the depolarisation is therefore in reverse and the PR interval is abnormally short. You usually do get a QRS after but sometimes it is blocked completely, and sometimes you get a RBBB pattern (RBBB has a longer refractory period).
You usually get a compensatory pause as the SA node is reset.
So the patient may feel a fast, extra beat, then a skip.
Considered normal! Can be very frequent eg bigeminy (every other beat is a PAC), and you can get runs (“couplets”). But if you have an abnormal heart already eg WPW, then it may be a trigger for a re-entrant tachycardia.