RELATIVE CLAUSE

A relative clause, also called an adjective or adjectival clause will meet three requirements.

First, it will contain a subject and verb.
Next, it will begin with a relative pronoun [who, whom, whose, that, or which] or a relative adverb [when, where, or why].
Finally, it will function as an adjective, answering the questions What kind? How many? or Which one?
The relative clause will follow one of these two patterns:

Relative Pronoun or Adverb + Subject + Verb

Relative Pronoun as Subject + Verb

Here are some examples:

• Which Francine did not accept

Which = relative pronoun; Francine = subject; did accept = verb [not, an adverb, is not officially part of the verb].

• Where George found Amazing Spider-Man #96 in fair condition

Where = relative adverb; George = subject; found = verb.

• That dangled from the one clean bathroom towel

That = relative pronoun functioning as subject; dangled = verb.

• Who played video games until his eyes were blurry with fatigue

Who = relative pronoun functioning as subject; played = verb.

PUNCTUATE A RELATIVE CLAUSE CORRECTLY

Many people find it difficult to decide when to use a comma before a relative clause and when this is unnecessary, but the rule is really rather simple. Punctuating relative clauses can be tricky. For each sentence, you will have to decide if the relative clause is essential or nonessential and then use commas accordingly.
Essential clauses do not require commas. A relative clause is essential when you need the information it provides.
Look at this example:

The children who skateboard in the street are especially noisy in the early evening.

Children is nonspecific. To know which ones we are talking about, we must have the information in the relative clause. Thus, the relative clause is essential and requires no commas.
If, however, we eliminate children and choose more specific nouns instead, the relative clause becomes nonessential and does require commas to separate it from the rest of the sentence.
Read this revision:

Matthew and his sister Loretta, who skateboard in the street, are especially noisy in the early evening.

Note:
When a relative clause adding extra information comes between a subject and a verb, it must have commas both before and after it These commas indicate that the relative clause could be removed and the basic sentence would remain the same.

Sources:
http://www.chompchomp.com/terms/relativeclause.htm
http://rwc.hunter.cuny.edu/reading-writing/on-line/relative_clauses.html