Use at the end of a statement (or a declarative sentence)

Ex. One of the figure skaters was Tara Lipinski.

Use at the end of a question (an interrogative sentence)

Ex. Did Gordon Parks write The Learning Tree?

Use at the end of an exclamation (an exclamatory sentence)

Ex. That's the biggest salad I've ever seen!

Use at the end of a request or a command (an imperative sentence)

Ex. Please give me the scissors. (a request)

Give me the scissors! (a command)