...as mentioned in another reply, the cars are fundamentally different (NASCAR hews closer to 20th-century stock-car archetypes, F1 closer to state-of-the-art open-wheel archetypes) and F1 is (at least nominally) a constructor's championship which competes on engineering, with all teams racing unique protoypes, while NASCAR is (at least nominally) a racing-team championship which competes strictly on drivers and tuning, with all teams racing identical silhouette cars...

...the truth is that as both series have evolved, the blend of constructor vs. racing-team competition has moved to converge somewhat toward the middle...

...the greater difference is that NASCAR is primarily raced on oval speedways while F1 is primarily raced on road circuits: the joke is that NASCAR goes fast while turning left but F1 starts, stops, and turns like real cars on a real road...

...again, though, both series have evolved to converge somewhere toward the middle: most F1 circuits are now run on dedicated racetracks which resemble twisty roads, and NASCAR now includes a few road-racing tracks in their series, too, to keep things interesting...one clear distinction is that only F1 includes real street circuits and only NASCAR races on oval speedways...

...culturally, NASCAR is the domain of american good-old-boys, and F1 the domain of european jet-setters, but there's a bit of crossover outside those core demographics...