There are four different types of sentences: Declarative, Interrogative, Imperative, and Exclamatory.
Interrogative Sentences are the most easy to identify of the four - they're questions. Anytime you see a question mark, you have an interrogative sentence.
Examples include: How's the weather today?, What's your name?, or If a tree falls in a forest but no one's around to hear to it, does it make a sound?.
Imperative Sentences are commands. The defining trait of imperative sentences is the implied "you". The reader understands that the "you" exists in the sentence as the subject, even though it's not actually there. Imperative sentences usually take an exclamation point but sometimes take a period.
Examples include: Finish your homework!, Close the door., or Stop!.
Exclamatory Sentences are sentences that show strong emotion, and thus, use an exclamation point, but are not commands. Sometimes, exclamatory sentences are not complete sentences, but are only one or two words of an outburst.
Examples include: Oh, my God!, Nooooooooooooooooooooooo!, or Boom!.
Declarative Sentences are your most basic sentences. Essentially, they're everything else. They end in periods and can either be very simple or very complex.
Examples include: It looks like it's going to rain today., Since the moon is made of cheese, Alice decided, at age ten, to be an astronaut, because she had never tasted a cheese she didn't like., or The dog barked.
Use all four types of sentences in your writing - remember that it's variety that makes a good writer, not the ability to create long, drawn out compound-complex sentences!