The answer to your question would be that this is an example of a compound sentence.
Explanation:
A compound sentence is a sentence that has at least two independent clauses joined by a comma, a semicolon or a conjunction. In this case, the two independent clauses have been joined by the coordinating conjunction <em>but. </em>
1) There is a swimming pool in our backyard (independent clause I)
2), <em>but</em> it contains no water (independent clause II)
