Brackets (also called parentheses) enclose extra information or explanations which interrupt the normal progression of the sentence.
Full stops, question marks or exclamation marks are usually put outside the brackets (unless the brackets enclose a complete sentence).
Additional Information in Brackets
Additional information is enclosed in brackets if the information is not essential for the understanding of the sentence.
Example: Connor (Amy's boyfriend) bought the tickets.
Depending on the importance attached to it, additional information can be enclosed in brackets, commas or dashes.
Brackets – not important
Connor (Amy's boyfriend) bought the tickets.
Commas – neutral
Connor, Amy's boyfriend, bought the tickets.
Dashes – emphasised
Connor–Amy's boyfriend–bought the tickets.
see also: → Additional Information in Commas
Abbreviations in Brackets
At the first mentioning of an organisation in a newspaper article, both its abbreviation and the spelled-out form are mentioned, one of which is enclosed in brackets.
He was an active member of the IOC (International Olympic Committee).
He was an active member of the International Olympic Committee (IOC).