A relative adverb is an adverb which introduces a relative clause. The English relative adverbs are: where, when, whenever and wherever.

The house where I live in is very small.
I will never forget the day when I met Jane.
Susie takes her cell phone wherever she goes.
I read books whenever I get time.

Relative pronoun

A relative pronoun is a pronoun which introduces a relative clause. Examples are: who, whom, which and whose. That is also sometimes used to introduce a relative clause. The use of a relative pronoun is usually optional in English. In informal English, it is usually omitted. In the following examples the optional relative pronoun appears in parentheses.

The necklace (which) my mother bought for me was very expensive.
The man (who) you were talking to is my father.

In the following examples, the relative pronouns who and whose are obligatory and cannot be omitted.

The woman who was sitting next to me is my aunt. (NOT The woman was sitting next to me is my aunt.)
The boy whose sister works with me is a good singer. (NOT The boy sister works with me is a good singer.)

Notes

A relative clause is a kind of subordinate clause which is attached to a noun within a noun phrase. A relative clause can be defining or non-defining. A defining relative clause is required to identify the noun it is attached to. In contrast, a non-defining relative clause is not required for identification; it only provides additional information.