What's the difference between parentheses and brackets?

Parentheses and brackets are similar in that they both enclose explanatory words and phrases or other incidental information. Brackets are generally used within parentheses or with quoted material:

Dextrose (usually sold in the form of fine, white granules

[grains] at specialty markets) is produced commercially by

putting starch in water mixed with dilute hydrochloric acid. 

John's reply stated: "I considered it my duty to inform my

supervisor [Fred Walters] of the discrepancy."