Basically, there are only two types of phrases that make up the Prolog language; a phrase can be either a fact or a rule. These phrases are known in Prolog as clauses. The heart of a Prolog program is made up of clauses.
More about Rules:
Find out that something is true by inferring it from other facts. A rule is a property or relation known to be true when some set of other relations is known. Syntactically, these relations are separated by comma