Answer: Hard as it may be to believe, I’m asked this question all the time. Even many knowledgeable collectors believe that a coupe is a 2-door car, and a sedan or a hardtop can be used interchangeably to describe a 4-door car. Neither of these beliefs is true.
As with anything, there are always exceptions to the rules, so please don’t send me angry letters telling me that my answer is incorrect because in the last three months of 1895 in a small French village in the foothills of the Pyrenees somebody named Pierre produced a car with 4 doors and called it a coupe. In the interest of full disclosure I am admitting in advance that my answer is most likely not technically 100% correct, but it is 99% correct as it applies to collector cars.
In order to understand all of the possible names for the types of cars, it will be helpful to first answer your question about pillars. Simply put, pillars are what hold the roof to the car. Looking at the car from the side, the pillars are described by letters, starting at the front of the car. The forward most one, often called the windshield pillar is the “A-Pillar.” The one in the middle is called the “B-Pillar” and the one at the rear is called the “C-Pillar.” A car with a “B-Pillar” is generally called a sedan. A car without a “B-Pillar” is generally called a hardtop.