Named after it's inventor, French mathematician George Boole, Boolean arithmetic is a small subset of discrete mathematics. Specifically, it is the study of systems in which a variable or expression may take on exactly one of two distinct values. Common definitions of these value pairs are: True/False, On/Off, and Yes/No. Boolean values are represented in UnrealScript with the 'bool' variable type.
see Boolean Operators on the [Operators] page for operators the produce and work with boolean values.
See Boolean_Algebra for an in-depth definition and some theory discussion.
inio: None of the other primitive types have pages of their own, why should this? If it is worth keeping around, shouldn't it be 'bool' to match the UnrealScript variable type?
Tarquin: you could just put this material on to Wikipedia and link to Boolean (but the original writer has to do this because of Wikipedia's GFDL.)