Defining relative clause

A noun can be modified in different ways: by an adjective, a noun or a phrase:

the best-selling book, a beautiful photo (adjective)
the history book, an art photo (noun)
the book with the most votes, a photo by Robert Capa (phrase)

Another way of modifying a noun is with a defining (also called a restrictive) relative clause:

the book I told you about last week
a photo which was taken by Robert Capa

This type of relative clause identifies or classifies the noun that it refers to.

An identifying clause defines who or what we are talking about and refers to a noun which is preceded by the definite article the:

This is the book I told you about last week. Remember?
Do you know the guy who is talking to Will over there?

A classifying clause describes what kind of person(s) or thing(s) we are talking about. It refers to a singular uncountable or plural countable noun without an article or a singular countable noun preceded by the indefinite article a or an:

I wrote my essay on a photo which was taken by Robert Capa.
Sometimes, I like listening to music that makes me sad.
I don't know any people who speak Esperanto.