Clauses used like this are called "relative clauses"

Now, we would like to introduce you two types of the relative clauses. The modification can be defining (or restrictive) and non-defining (or non-restrictive). That is, the head can be viewed as a member of a class which can be linguistically identified only through the modification that has been supplied (defining):
Eg: …as can the men of towns who are more to the manner born.
…and unperceived farmer who were alone its spectators…
There are several grammatical differences between the two kinds of relative clause. There are also stylistic differences: non-defining clauses are generally more formal, and are less frequent in informal speech.
Or the head can be viewed as unique or as a member of a class that has been independently identified, eg. in a preceding sentence. Any modification given to such a head is additional information which is not essential for identifying the head, and we call it non-defining:
Eg: The ring of the sheep-bell, which had been silent during his absence.
But there is a way some men have, rural and urban alike, for which the mind is more…

Bathsheba Everden, who wore no riding-habit, looked around for a moment.
This sentence has a non-defining relative clause since Bathsheba Everden´s identity is dependent of whether or not she has the riding-habit, though the information on her present manners may be useful enough. In this example, the modification is inherently non-defining, since the heads in question – being treated as unique – will not normally admit definition. But any head can be non-defining modified.

Postmodification
Explicitness:
Premodification is in general to be interpreted in terms of postmodification and its greater explicitness. It will therefore be best to begin our detailed study of noun-phrase structure with the forms of postmodification. Explicitness in postmodification varies considerably, however. It is greater in the finite relative clause:
Eg: Gabriel had always known that when the fire was lighted the door must be kept open.
than in the non-finite clause:
Eg: Gabriel had always known when the fire was lighted the door must be kept open.
Part of the relative clause´s explicitness lies in the specifying power of the relative pronoun. It is capable A/ of showing agreement with the head and
B/ of indicating its status as an element in the relative clause structure.