Once upon a time, the difference between a sedan and a coupe was straight forward: a sedan had four doors and four seats, whereas a coupe was shorter and usually featured just two doors. Over the years, however, automakers have blurred the lines between the two in the name of marketing and design.
There are still a few key differences that can help you distinguish between the two, though. A sedan is usually longer with room for three people in the rear seat and frequently has greater engine displacement options to help move their larger size. Like an SUV, these vehicles are designed for passenger seating and often feature a sizable trunk. When you think sedan, think Toyota Camry or Nissan Maxima.
A coupe, on the other hand, tends to fall more to the sportier side of things. They have a shorter wheelbase, a lighter overall curb weight and only room for two people in the rear. Performance over passenger capability tends to be the focus of these cars. Some good examples of a classic coupe would be the Ford Mustang and the Mazda RX-8.
As mentioned previously, it is a common assumption that the main difference between a sedan and a coupe is the number of doors. But it is not unheard of for manufacturers to sell two-door sedans and four-door coupes these days - sometimes even within the same model line. The 2011 Cadillac CTS, for example, was offered as a four-door or two-door car. Both a sedan and coupe share the feature of having a trunk, but the sporty nature of the Coupe means that vehicle will likely have a smaller rear trunk than the Sedan option. Again, however, this is not always the case: a 2018 Ford Mustang Coupe has 15.1 cubic feet of trunk space, where the 2018 Nissan Maxima Sedan has 14.3.
At the end of the day, the reality is that the terms coupe and sedan have become more marketing jargon than actual definable automotive designations. There are no hard-set rules on just what a sedan or a coupe really is. The important thing is that you shop around and find a car that fits your needs - find a style you like, take it for a test drive and decide if that is the right car for you.