We may call them plug doors, and say that they are larger than their openings but that is only partly true. Most are only wider than the opening, not taller. The only shape that cannot pass through its own opening is round. That is why manhole covers are round - it is the only shape that cannot fall down the hole.
A "true plug" type door will be wider than the opening but not taller. To open, you typically will pull it inward, then rotate so that it is, in effect, narrower than the opening. Or, better yet, it will roll up inside the fuselage out of the way, as described by Dairbus above.
What is often called a "semi-plug" is the passenger door on an A-320 for example. What makes it plug-type is that lugs on the door rest directly against hard points on the door frame and the pressure differential holds it securely against those metal lugs. Like a true plug type door, if all the latch mechanism failed in flight, pressurization would hold it in place.
Non plug types, like most aircraft cargo doors latch up to the exterior, in effect. In flight if the latches failed, it would fly away.
Now a trivia question: On the DC-9 in passenger configuration, one door in the pressure vessel is NOT a plug-type door. Which one? Bonus points, what safety feature does it incorporate to compensate for this?
Happiness is not seeing another trite Ste. Maarten photo all week long.