Brackets

Brackets ( ) can be used in the same way as commas (,) or dashes (-) to separate a word or phrase that has been added to a sentence as an explanation or afterthought.

The word or phrase inside the brackets is called a parenthesis.

My birthday cake was chocolate (which is my absolute favourite) with chocolate icing and chocolate buttons on top as well.

http://www.oxforddictionaries.com/words/punctuation