A "relative clause" is a string of words that contains a noun and a verb. Relative clauses can not stand on their own as a sentence and must begin with a "relative pronoun".

This is one of the most difficult aspects of English. Consider:

• The bed I was lying on. (implies an answer to a previous question)
• The bed that I was lying on... (implies a continuation)
• The bed which I was lying on... (also implies a continuation)

Consider these two:

(1) The builder, who erects very fine houses, will make a large profit. (non-restrictive)

(2) The builder who erects very fine houses will make a large profit. (restrictive)

On the first, he will make a large profit, and it gives the reason. In the second, the profit only comes if he builds well. The profit is conditional upon his performance.