Malware, short for "malicious software," is a broad category of software which is installed without your permission to damage your PC or spy on your computer activities. The most common types of malware are trojans, worms and viruses.
A trojan is a program that seems harmless or even productive, but is designed to exploit the system where it is run. Worms are self-replicating programs that can be spread through networks of machines with or without the user's intervention. A virus attempts to spread itself by attaching to a host program, and can damage hardware, software or data in the process. Other types of malware are malicious active content, rogue programs and dialers. Trackware is a newer variation of malware that compromises your privacy by tracking, storing and analyzing your browsing patterns. It includes adware, spyware, key loggers and tracking cookies.
Malware is most commonly delivered through the Internet and by email messages that misrepresent a program's purpose and function. Thirty-one percent of dangerous websites feature adult content and can be accessed by mistyping URLs or accidentally clicking unknown links resulting in unintentional exposure to objectionable material. Peer-to-peer (P2P) networks in which users can transfer files directly from one computer to another is a significant risk for malware and adware.
Symptoms of malware infection are pop-up ads, general slowdown, or browser redirects to unwanted websites. Malware can disrupt your work or the normal operations of the system by causing a denial of service, corrupting data, taking up system resources and consuming network bandwidth. In addition, your computer may not shut down or restart.