A complex sentence is a sentence that contains one independent clause and at least one dependent clause, joined by a subordinating word.

A compound sentence is a sentence that contains at least two independent clauses, joined by a semicolon or a comma and a conjunction.

A compound-complex sentence is a sentence that has at least two independent clauses and at least one dependent clause.

A simple sentence is a complete thought that contains at least one subject and one verb.

An independent clause is a group of words that can stand alone as a sentence; it has a subject and a verb and expresses a complete thought.

A dependent clause is a group of words that cannot stand alone as a sentence; it has a subject and a verb but does not express a complete thought.

A subordinating conjunction is a word that connects a dependent clause to an independent clause, indicating a relationship between the two.

Although it was raining, we decided to go for a walk.

I wanted to go for a walk, but it started to rain.

Although it was raining, I went for a walk, and my friend joined me.