There are two types of relative clauses: defining and non-defining.
We use:
defining relative clauses to identify the person or thing we are talking about:
A doctor is a health specialist who helps people in hospitals or clinic.
non-defining relative clauses to give extra information about the person or thing, which is not necessary to identify this person or thing and can be left out:
A reporter, who finds events to speak about, travels all over the world.
We can't use pronoun that or omit the relative pronoun in non-definite relative clauses.
We use special type of non-defining relative clauses to add a comment to what was said in the first part of the sentence:
In this clauses, we always use a comma and the relative pronoun which (not what).
Instead of a full relative clause we can sometimes use a participle clause.
We use:
a present participle to say what the person/thing is doing.
You can see many people wearing trousers everywhere.
a past participle to say what is/was done to the person/thing mentioned:
I was looking at the walls covered with photos of clients.