The four kinds of sentences are labeled by purpose. They are declarative, imperative, exclamatory and interrogative.
A declarative sentence is a statement. It gives information about something.
I like donuts.
Declarative sentences end in periods.
An exclamatory sentence is similar to a declarative sentence in that it gives information. It also conveys strong emotion.
I love donuts!
Exclamatory sentences always end with exclamation points.
An interrogative sentence is a question. It is asking for information, rather than giving it.
Do you like donuts?
Interrogative sentences always end with a question mark.
The imperative sentence is a command. It tells a person to do something.
Give me a donut.
The imperative sentence can end with either a period or an exclamation point, depending on whether the command has strong emotion to it or not. The above example is just a normal command, but if you had strong emotion you might use an exclamation point.
Get out of the way!
Imperative sentences are commands, and can end in either a period or an exclamation point.
Using all four kinds of sentences can really improve your writing. It makes your writing more interesting by making it more varied. Just make sure you use the right end punctuation!