dumb - stupid , not very bright. Dummy - this is an Americanism, not used in British English - in British English, a dummy is what you give to a baby to calm them - Americans call these 'pacifiers'.

They mean the same. Dummy is the softer version.

Dumb is a descriptive word, Dummy is a noun (a stupid person), also what we call a 'pacifier' in Britain.

I would make it as simplest as I can demonstrating the following examples: It's a dumb donkey. It's a dummy.

Note that both of these words have standard meanings other than the colloquial sense of 'stupid'. 'Dumb' means speechless. And 'dummy' means an imitation of something used as a substitute - check out the phrases 'dummy run' and 'crash test dummy' for example.