A relative pronoun is used:

• to represent a preceding noun or pronoun, called its antecedent.
• to establish a connection with a dependent clause.

For example

In the sentence:
 Those who wish for favors must assist others.
who is a relative pronoun. Its antecedent is those, and it connects the verb phrase wish for favors to the identity of those.

Relative pronouns when used only as relatives follow their antecedents.

For example

All that I have is yours.

When a relative pronoun is used both as a limiting adjective and a pronoun, the pronoun always precedes its antecedent.

For example

I will give you what money I have.

The following distinctions will show the difference between a relative and a personal pronoun.
The relative refers to an object always known, and either previously mentioned, or so clearly implied as to need no mention.

Relative pronouns do not change their form to represent person.

The personal pronouns refer always to an object known. In the third person, it must refer to an object previously mentioned. In the first and the second person, it can refer to an object not previously mentioned.
    
The personal pronouns have a distinct form for each grammatical person and number:

• I and we for the first person
• you for the second person
• he, she, it, and they for the third person.

The essential difference is seen

• in the relations that they denote.
• in their use in construction.

The personal pronoun may represent the subject of an independent sentence, that is, one expressing a thought.
    
The relative pronoun shows a dependent adjective relation.

For example

In the following:
He is present.
which is important
the first is a complete sentence. The second needs some word or phrase upon which depends.

Simple Relatives

The simple relative pronouns are

• who
• which
• that
• what

Who is used to represent persons, which and what represent animals or things, and that can represent persons, animals, or things.
The antecedent of a relative pronoun is not only the word for which the pronoun stands, but also the leading or antecedent term of a relation of which the clause introduced by the relative pronoun is the subsequent term.
The antecedent of a relative pronoun is that upon which the relative clause depends.
Who, which, and that normally refer to definite antecedents.

For example

the man who came
the horse that died
the debt that was paid

What may refer either to a definite or an indefinite antecedent.

For example

In the sentence:
I gave him what money he wanted.
the antecedent of what is the definite money.
In the sentence:
I gave him what he wanted.
The antecedent  of what  is something indefinite.