A dumbphone is basically the opposite of a smartphone. It can only do some basic functions, like calling, texting, and taking pictures. Generally has lower specs than a smartphone in terms of screen size, wireless connectivity, storage space etc.
This is a near-exhaustive list of smartphone uses. If your phone doesn’t use something on this list it is probably a dumbphone:
• Window Mobile
• Palm OS
My wife currently has a dumbphone, so I might get her a smartphone for her birthday.