Booleans represents the domain of booleans, as in x Booleans.
The domain of booleans is taken to consist of the symbols True and False.
x Booleans evaluates immediately if x is explicitly True or False.
Simplify[expr Booleans] can be used to try to determine whether an expression is boolean, with no undetermined variables.
Boolean is output in TraditionalForm as .
See The Mathematica Book: Section 3.3.10.
See also: Element, Simplify, True, False, Integers.