Parentheses set off material not essential to the meaning of the text.
They are used for asides and explanations when the material is not essential or if it is made up of more than one sentence.
Parentheses may contain a complete sentence or sentences.
Example: He had to go through the usual process to get his bus driver's license (police and FBI check, reference check, motor vehicle check, written exam, mechanical test, and driving test).
(This could be set off by a colon for more emphasis since it is a list or by a dash for strong emphasis. But since the sentence says "the usual process," there is no need to emphasize anything.)
See also Dashes and Capitalizing and Punctuation in Parentheses.