A simple sentence has only one clause. The cat is sleeping.
A compound sentence has two or more clauses. These clauses are joined together with conjunctions, punctuation, or both. The dog is happy, but the cat is sad.
A complex sentence has one clause with a relative clause. The dog, which is eating the bone, is happy.
A complex-compound sentence (or compound-complex sentence) has many clauses, at least one of which is a relative clause: The dog, which is eating the bone, is happy, but the cat is sad.
Sentences have different purposes:
A declarative sentence, or declaration, is the most common type of sentence. It tells something. It ends with a full stop . (The dog is happy.)
An interrogative sentence, or question, asks something. It ends with a question mark ? (Is the dog happy?)
An exclamatory sentence, or exclamation, says something out of the ordinary. It ends with an exclamation mark ! (That dog is the happiest dog I have ever seen!)
An imperative sentence, or command, tells someone to do something. (Give the dog a bone.)