A sentence is a group of words arranged in a specific order which makes complete sense.

There are different kinds of sentences and they are

Assertive or declarative sentences
Interrogative sentences
Imperative sentences
Exclamatory sentences
Optative sentences

Assertive sentences
Sentences which make simple assertions or statements are called assertive or declarative sentences. Examples are:

It is raining.
The child is going to school.
I get up early in the morning.
I like reading.

Assertive or declarative sentences may be positive (affirmative) or negative. Sentences which give a positive or affirmative sense are called affirmative sentences.

Honesty is the best policy.
Barking dogs seldom bite.
I have been to Canada.

Sentences which give a negative meaning are called negative sentences.

She will not listen to me.
She should not hate anybody.

Interrogative sentences
Sentences which ask questions are called interrogative sentences.

What is your name?
Where do you live?
Who is your father?
Can you solve this problem?

Imperative sentences
Sentences which express orders, commands, requests, advice, proposals or suggestions are called imperative sentences.

Leave this place at once. (Order)
Get lost. (Order)
Please help me. (Request)
Work hard. (Advice)
Let’s go for a walk. (Suggestion)

Exclamatory sentences
Sentences which express some strong feelings or emotions such as joy, sorrow, regret, surprise, wonder etc., are called exclamatory sentences.
What a marvelous sight!
How beautiful the flower is!
Hurrah! We have won.
Alas! She is no more.

Optative sentences
Sentences which express an ardent wish, prayer, curse etc., are called optative sentences.
May you live long!
May God help you!
Notes:
Assertive and imperative sentences are followed by a full stop. Interrogative sentences are followed by the sign of interrogation or question mark (?). Exclamatory and optative sentences are followed by the sign of exclamation (!). Note that in exclamatory sentences, sometimes, the interjection alone is followed by the exclamatory mark.