Tortious interference with contractual relations is the most common of the business torts. Under this cause of action, a plaintiff can seek damages for the loss of a contractual relationship as a result of improper interference by a third party. Punitive damages are also available in the case of outrageous or malicious conduct. Importantly, someone cannot interfere with his/her own contractual relationship. This means that a tortious interference claim cannot be used to replace a breach of contract claim.
So is it tortious interference with contractual relations if a business competitor tells your client that his services are better than yours, or points out that your products have a known flaw or limitation? This conduct certainly is intended to interfere with your relationship with clients/customers. However, it is not tortious (improper). Stating opinions or true facts will almost never be considered an improper interference.
But compare a situation where the interference is an intentionally false and disparaging statement about your business. Also, consider a scenario where a business competitor offers compensation to a person to disclose your trade secrets or violate a non-disclosure agreement. These are improper interferences for which damages may be sought under the rubric of tortious interference.