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.