ASCII 91 and 93 (and hence U+005B and U+005D). Used in matching pairs to denote stage directions in a play or additions to a quotation. In programming languages with C-like syntax, square brackets typically enclose array subscripts. In mathematics, their most frequent use is to denote closed intervals of real numbers. On Urban Dictionary, because they're used for markup and there are no escape sequences, opening square brackets can never appear in the text of a definition.
The standard QWERTY keyboard makes square brackets easier to type than parentheses, which is pretty silly considering how much more common parentheses are.