They probe a group of machines for open ports to learn which services are running.They are maliciously formed code segments used to replace legitimate applications.They target specific individuals to gain corporate or personal information.They are infected machines that carry out a DDoS attack.

The first sentence describes the actions of a network scanner or a port scanner. These are tools used by both system administrators and hackers to discover open ports on a machine and identify running services. In a malicious context, this information can be used to find potential vulnerabilities to exploit.

The second sentence refers to Trojan horses. These are malicious programs that disguise themselves as legitimate software. Users are tricked into loading and executing Trojans on their systems. Once activated, the authors can access sensitive information or take control of the system.

The third sentence describes a method often used in spear phishing attacks. Spear phishing is a targeted form of phishing where the attacker knows specific details about their victim and creates a personalized message to trick the victim into revealing confidential information.

The fourth sentence refers to botnets. A botnet is a network of private computers infected with malicious software and controlled as a group without the owners' knowledge. The botnet can be used to carry out a Distributed Denial of Service (DDoS) attack, where multiple machines target a single system to overwhelm it and disrupt its services.