When two people have a contract, a third party may try to interfere with the contractual relationship.  For example, if a vendor has a contract to supply a customer, a competitor of the vendor may encourage the customer to breach his contract and become a customer of the competitor.  Under the right circumstances, the injured vendor may recover the resulting damages.

This cause of action has two subcategories: 1) tortious interference with contract, and 2) tortious interference with prospective economic advantage.  Tortious interference with contract deals with a situation where A and B have a contract, and C interferes with it.  The elements are: (1) the existence of a valid contract between plaintiff and a third party; (2) the defendant's knowledge of that contract; (3) the defendant's intentional procuring of the breach, and (4) damages. 

Thus, it's not necessary to allege facts showing that the defendant's conduct was tortious or criminal.  In the example given above, merely persuading the customer to breach his contract with the first vendor would suffice. 

By contrast, to allege interference with prospective economic advantage, the plaintiff must allege conduct which is an independent tort or criminal act.  This includes, for example, bringing a baseless civil action, misrepresenting the facts or threatening violence. 

The rationale for the difference is that, where there is a contract, there is an interest that the law seeks to protect, so the threshold for bringing a law suit is lower.  By contrast, where there is no contract, there is no such interest, and the law sets a higher threshold that must be surmounted.