informal

Some firefighters never meet the people they save.

formal

Some firefighters never meet the people whom they save.

Here are some more examples of adjective clauses:

the meat which they ate was tainted

This clause modifies the noun "meat" and answers the question "which meat?".

They're talking about the movie which made him cry

This clause modifies the noun "movie" and answers the question "which movie?".

They are searching for the student who borrowed the book

The clause modifies the pronoun "student" and answers the question "which student?".

Did I tell you about the author whom I met?

The clause modifies the noun "author" and answers the question "which author?".

2. Restrictive & non restrictive clauses

Do the following pairs of sentences mean the same thing?

1a My uncle, who lives in London, is very rich.

2b My uncle who lives in London is very rich.

2a The policies, which were unpopular, were rejected by the voters.