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.
A Shackteau is a humble, weather-beaten, structurally questionable shelter located in a spectacular or highly coveted place—Wales, Jackson Hole, Sun Valley, Crested Butte, coastal Maine, the Alps—where the building itself may be worth almost nothing, but the dirt, view, access, and mythology make it absurdly valuable.
In use:
Shackteâu - We thought it was an abandoned shed until the realtor called it a rare alpine Shackteâu with unobstructed views and listed it for $2million.