Types of Spyware and Viruses

Computer systems are vulnerable to software designed with malicious intent--referred to as malware by the computer security industry and categorized by infiltration method and payload. While infectious malware is capable of spreading between systems and networks without direct intervention, non-infectious types must rely on concealment and deception to gain access to a system. A payload is a script, process or utility designed to be deployed upon successful infiltration. Payloads may be designed to collect private data and passwords, vandalize files or make changes to system and security configurations.
Spyware
Malware used to collect information about a computer's user is referred to as spyware and is designed to avoid detection while gathering and transmitting data. Spyware has traditionally been used to gather personal information, passwords, and web browsing habits. These programs can be deployed by viruses, worms and trojan horses; but are most commonly bundled with shareware.
Adware
Spyware specifically designed to display advertisements is referred to as adware. Infected systems and programs are often significantly modified in order to present advertisements to users. Web browsers are frequently targeted for infection, as the offending program can track internet usage, launch pop-up advertisements and alter configuration settings. Some adware may exploit vulnerabilities that leave systems susceptible to attacks by other malware.
Viruses
A classic computer virus is capable of self-replication on an infected machine and often spreads throughout the networks and writable media it's connected to. Viruses attach themselves to uninfected files and programs to gain the necessary permissions required to execute and store malicious code. Word processors and productivity applications often include a basic programming language, permitting users to write capability enhancing scripts and utilities. These macro languages are often used to write viruses capable of exploiting system privileges granted to the application.
Worms
Computer worms are designed to spread and multiply, but usually avoid causing overt alterations to their individual host systems. Most worms attempt to spread across networks by infiltrating connected computers with specific security vulnerabilities. Worms are commonly used to give a single person remote control over a large number of vulnerable computers---a botnet. These infected computers are called "zombies" and are typically made to perform denial of service attacks and send e-mail spam.
Trojans
Classified by their unique delivery system, Trojan Horses rely on social-engineering to successfully infiltrate a target system. Trojans attempt to fool users into downloading, installing, or executing malware--often employing creative methods of deception. Some may disguise themselves with elaborate and convincing installers, masquerade as a beneficial applications or piggyback on trusted software. Common payloads consist of keystroke loggers, password stealers, botnet software agents and destructive code.