Defining relative clauses.  They are used to identify who or what we are talking about: 

The man who phoned a moment ago wanted to speak to Susan.

  If we said, "the man wanted to speak to Susan," the person we are talking to would not understand which man we are speaking about.

Non-defining relative clauses. They do not define their antecedent. They only give more information about it but this information is not essential to understand who or what we are talking about:

This aqueduct, which was built in the first century AD, is still in use.

   We can say, This aqueduct is still in use, and there is no doubt what we are talking about. A non-defining relative clause does not define its antecedent. It only gives us more information about it.