There are two types, defining and non-defining. Defining add essential information to tell us what we are talking about, non-defining add extra information.
Example
‘They demolished the house where I used to live' is an example of a defining relative clause.
In the classroom
Effective activities to practise relative clauses include writing definitions for crosswords and competitions to make sentences as long as possible.
Further links:
https://www.teachingenglish.org.uk/article/revising-defining-relative-clauses
https://www.teachingenglish.org.uk/article/left-handers
https://www.teachingenglish.org.uk/article/unsung-heroes