Sentences may be classified according to the purpose of the sentence.

Sentence classification by purpose

A declarative sentence is used to make a statement.
An interrogative sentence is used to pose a question.
An imperative sentence is used to give a command or to implore or entreat.
An exclamatory sentence is used to express astonishment or extreme emotion.

Most of the sentences we speak or write are declarative sentences.

Declarative Sentences

It's lunch time.
We are going to the game on Friday.
My car is out of gasoline.
My parents keep telling me that I should make good grades so I can get a job or go to college.

We frequently ask questions, perhaps not as frequently as we should.

Interrogative Sentences

What time does the movie start?
How many people from your graduating class went to college?
Is there a reason why these dirty clothes are in the middle of the floor?
What are they serving in the cafeteria today?

People who have authority use imperative sentences. Sometimes, people who don't have authority use imperative sentences. The results may differ.

Imperative Sentences

Wash the car.
Clean up your room.
Martin, report to the counselor.
Please donate to the community charity fund.

We say that sentences must have a subject and a verb. Note that some of the above sentences do not seem to have a subject. The subject is implied, and the implied subject is you. You wash the car. You clean up your room. You is a second person pronoun. It isn't possible to make a command statement in first person or third person.

Exclamatory sentences are rarely used in expository writing. Spoken exclamations are often a single word or an incomplete sentence. Grammarians indicate that formal exclamatory sentences begin with the word what or with the word how. Most of the exclamations we encounter are informal.

Exclamatory Sentences

What a beautiful night!
How happy we were when the dawn came and our flag was still there!
What did you do to your hair! (exclamation formed as a question)
I just won 500 dollars! (exclamation formed as a declarative sentence)

How do you know if a sentence is a question? Well, according to commedians Bud Abbot and Lou Costello, it depends on the punctuation mark.
"Who's on first."

Source: http://www.iscribe.org/english/sent.html