A threat actor, also called a malicious actor, is an entity that is partially or wholly responsible for an incident that impacts – or has the potential to impact -- an organization's security.
In threat intelligence, actors are generally categorized as external, internal or partner. With external threat actors, no trust or privilege previously exists, while with internal or partner actors, some level of trust or privilege has previously existed. The actor may be an individual or an organization; the incident could be intentional or accidental and its purpose malicious or benign.
External actors are the primary concern of threat intelligence services not only because they are the most common, but also because they tend to be the most severe in terms of negative impact. Such threat actors are sometimes categorized as either being commodity or advanced. A commodity threat actor launches a broad-based attack hoping to hit as many targets as possible, while an advanced threat actor targets an organization, often seeking to implement an advanced persistent threat (APT) in order to gain network access and remain undetected for a long time, stealing data at will.
Another type of external threat actor is the hacktivist. Hacktivist groups such as Anonymous use many of the same tools employed by financially-motivated cybercriminals to detect website vulnerabilities and gain unauthorized access or carry out distributed denial-of-service (DDoS) attacks. The motivation of most hacktivists is to gain access to sensitive information that will negatively impact the reputation of an individual, a brand, a company or a government.
Learn more about commodity vs. advanced threat actors: