Subordinate clauses are dependent upon main clauses or other dependent clauses. They cannot stand on their own.
Often they begin with words (subordinating conjunctions, relative pronouns or question words) that connect them to the independent clause.

Relative clauses are subordinate clauses that modify a noun or pronoun in the independent clause. Hence they refer back to a word in the main clause. Usually the relative clause comes directly after the word it modifies and is separated by a comma. If the independent clause continues after the relative clause, then a comma comes after the relative clause as well.

Examples:
Der Mann hat ein blaues Auge. Fußballfans haben den Mann geschlagen.
Der Mann, den Fußballfans geschlagen haben, hat ein blaues Auge.