A demonstrative is a pronoun or adjective which points out which item is being referred to.
In English there are only four demonstratives: this, that, these, and those.
A demonstrative pronoun is a demonstrative used in the place of a noun.
A demonstrative adjective is a demonstrative used to modify a noun.
Demonstrative pronoun: May I see that?
Demonstrative adjective: May I see that book?