It's called a relative clause. It is also called an adjective clause because it modifies the noun, "girl."
first it is a dependent clause, since it cannot stand by itself. it is also a relative clause, since Relative clauses are dependent clauses introduced by a Relative Pronoun (that, which, whichever, who, whoever, whom, whomever, whose, and of which). it is also restrictive, as A nonrestrictive clause is not essential to the meaning of the sentence; it can be removed from the sentence without changing its basic meaning.
If you need further help, type in "clauses" into google search and click on the first search you see.
thanks :) I will remember that.