Relative clause

A relative clause is a kind of subordinate clause that contains an element whose interpretation is provided by an antecedent on which the subordinate clause is grammatically dependent; that is, there is an anaphoric relation between the relativized element in the relative clause, and the antecedent on which it depends.

Typically, a relative clause modifie a noun or noun phrase, and uses some grammatical device to indicate that one of the arguments within the relative clause has the same referent as that noun or noun phrase. For example, in the sentence I met a man who wasn't there, the subordinate clause who wasn't there is a relative clause, since it modifies the noun man, and uses the pronoun who to indicate that the same "man" is referred to within the subordinate clause (in this case, as its subject)..

In many European languages, relative clauses are introduced by a special class of pronouns called relative pronoun; such as who in the example just given. In other languages, relative clauses may be marked in different ways: they may be introduced by a special class of conjunctions called relativizers; the main verb of the relative clause may appear in a special morphological variant; or a relative clause may be indicated by word order alone. In some languages, more than one of these mechanisms may be possible.

Restrictive and non-restrictive

Bound relative clauses may or may not be restrictive. A restrictive, or defining, relative clause modifies the meaning of its head word (restricts its possible referent), whereas a non-restrictive (non-defining) relative clause merely provides supplementary information. 

For example:

The man who lives in this house has not been seen for days. This (who lives in this house) is a restrictive relative clause, modifying the meaning of man, and essential to the sentence (if the clause were omitted, it would no longer be known which man is being referred to).

The mayor, who lives in this house, has not been seen for days. This is a non-restrictive relative clause, since it provides supplementary information about the mayor, but is not essential to the sentence – if the clause were omitted, it would still be known which mayor is meant.

In speaking it is natural to make slight pauses around non-restrictive clauses, and in English this is shown in writing by commas (as in the examples). However many languages do not distinguish the two types of relative clause in this way. Another difference in English is that only restrictive relative clauses may be introduced with that or use the "zero" relative pronoun.

In colloquial speech, a non-restrictive relative clause may have a whole sentence as its antecedent rather than a specific noun phrase; 

for example:

The cat was allowed on the bed, which annoyed the dog.

Here, the context of the sentence (presumably) indicates that which refers not to the bed or the cat but to the entire proposition expressed in the main clause, namely the circumstance that the cat was allowed on the bed. Such constructions are discouraged in formal usage and in texts written for nonnative speakers because of the potential for ambiguity in parsing; a construction more accepted in formal usage would be The cat's being [or having been] allowed on the bed annoyed the dog.

Source : https://en.wikipedia.org/wiki/Relative_clause