Xor[, , ... ] is the logical XOR (exclusive OR) function.
It gives True if an odd number of the are True, and the rest are False. It gives False if an even number of the are True, and the rest are False.
Xor[, , ... ] can be input in StandardForm and InputForm as ... . The character can be entered as xor or \[Xor].
Xor gives symbolic results when necessary, applying various simplification rules to them.
Unlike And and Nand, Or and Nor, Xor must always test all its arguments, and so is not a control structure, and does not have attribute HoldAll.
See The Mathematica Book: Section 1.5.6.
See also: LogicalExpand, Mod.