A compound sentence consists of two independent clauses joined by a coordinating conjunction:
"I washed the car and Bill took out the garbage."
Coordinating conjunctions include and, but, or, so, yet and for. The two clauses have equal status in the sentence.
If one short sentence is dependent upon another, then they can be combined into a complex sentence, using a subordinating conjunction:
"Because you have foiled my plans once too often, I shall be forced to eliminate you, Mr. Bond."
Subordinating conjunctions include because, although, after, if, since, though and before.