What is the difference between marinara and spaghetti sauce?
Marinara is a style / kind of a sauce that originated in Napoli usually made with tomatoes, garlic, herbs, and onions. A spaghetti sauce only says where to sauce is used (obviously on spaghetti) but doesn't say anything about what the sauce is exactly like. 

There are many dishes which are basically spaghetti + sauce:


Spaghetti alla marinara – which literally translates to "spaghetti mariner's style"
Spaghetti aglio e olio – "spaghetti with garlic and oil", originated in Napoli
Spaghetti alla puttanesca – "spaghetti whore-style"
Spaghetti alla Carbonara – "spaghetti coal worker's style"


and, uhmmm...


Spaghetti with meatballs – an Italian-American invention