Which noun is Bob?

The noun 'Bob' (capital B) is a proper noun, the name (usually a nickname) of a specific person.

The noun 'bob' (lower case b) is a common noun, a general Word for an up and down movement; woman's or child's short haircut; a weight hanging from a fishing line; a Word for a thing.

The Word 'bob' is also a verb: bob, bobs, bobbing, bobbed.