This week I heard some people use “popular” and “famous” interchangeably. There are lots of overlaps, certainly — but they don’t mean exactly the same thing.
“Popular” means well-liked; “famous” means well-known.
To wit:
That girl from high school who everyone liked was popular, but she wasn’t famous.
Pizza is a popular food. Pizza Hut is a famous pizza franchise. Comet Ping Pong is a not-so famous pizza franchise, but is well-known in certain circles.
Darth Vader, Hitler, and Metallica are all famous, but their levels of popularity have varied over the years.
Incidentally, “infamous” means famous for a bad reason. Think Darth Vader, Hitler, and Metallica.