a clause is an expression formed from a finite collection of literals . That is, it is a finite disjunction or conjunction of literals, depending on the context.
horn clause is a disjunction of literal in which at most one positive literal.
~X1 V ~X2 V……..V~Xn V Y
Every Horn clause can be written as an implication whose premise(base) is a conjunction of positive literals and whose conclusion is a single positive literal. E.g. lawyer(x) rich(x)
Any Horn clause therefore belongs to one of four categories:
read complete article click here