Stuff is less formal, and you can not put a number in front if it. You can say : I own 3 things. You can't say : I own 3 stuff. Stuff is very general and the exact amount is unknown. For example, She was telling me some stuff about math. (We don't know exactly how much)

I need to pack my things/stuff I can't find my things/stuff I lost my things/stuff Stuff=less formal