Relative Clause

A relative clause is a subordinate clause that modifies a noun phrase, most commonly a noun. For example, the phrase "the man who wasn't there" contains the noun man, which is modified by the relative clause who wasn't there. A relative clause can also modify a pronoun, as in "he to whom I have written", or a noun phrase which already contains a modifier, as in "the black panther in the tree, which is about to pounce". The complete phrase (modified noun phrase plus modifying relative clause) is also a noun phrase.
In many European languages, relative clauses are introduced by a special class of pronouns called relative pronouns; in the previous example, who is a relative pronoun. 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.
A relative clause is a clause which describes the referent of a head noun or pronoun. It often restricts the reference of the head noun or pronoun. A relative clause is not necessarily a constituent of the noun phrase containing the head noun it modifies.
Example :

• The plumber arrived who we had called earlier. (Who we had called earlier has been extraposed from its normal position after plumber and is not a member of the noun phrase containing plumber.)
• The man who went
• Passengers leaving on Flight 738 

 
Source :
http://en.wikipedia.org/wiki/Relative_clause
http://www.sil.org/linguistics/GlossaryOfLinguisticTerms/WhatIsARelativeClause.htm