differemce between demonstraive pronoun and relative pronoun

A Demonstrative Pronoun points to the object which it refers.

Example: This is my house.

Here, the pronoun 'this' points to the object that is 'house'.

A Relative Pronoun points or refers to the noun going before it, which is called its antecedent.

Example: I have bought the book that I wanted.

Here, the pronoun 'that' relates to the noun going before it i.e. to 'the book'