What Is A Virus?

Viruses are self-executing, self-replicating programs. They alter the way a computer operates without the knowledge or permission of the user.  When activated, viruses may damage files, cause erratic system behavior, or display annoying messages. The ability to self-replicate differentiates viruses from Trojan horses, worms, and other virus-like programs.

Active Content

Small programs that get downloaded to a user’s machine to add functionality to Web pages, such as Java applets and ActiveX controls. Blocking all active content when browsing the Web is inconvenient because some trustworthy sites use it to provide useful services to customers. But allowing all active content can be dangerous because a Web site may use it with malicious intent to gain open access

Cookies

Small files that Web sites save to a local computer. After a cookie file is saved locally, when you go back to a Web site, the first thing the site does is check to see if there’s a cookie in the computer. From the information in the cookie the site deduces the identity of the computer accessing the Web site. When a site knows who you are, it can present you with customized content to match your preferences.

Mobile Code

Web-based applications (usually in the form of Java Applets, Visual Basic scripts, or ActiveX controls) that are generally intended to enhance the appeal, usability, or interactivity of a web page. They can be as “ornamental” as spinning icons on a web page, or as integral as the script that enables you to fill out and submit forms. Because these are actual pieces of computer code that automatically execute upon downloading of web pages, they can be exploited to deliver malicious “payloads.”

What Is A Trojan Horse?

Trojan horses are malicious programs disguised as something benign. They've been known to pose as games, utilities, and email attachments. Once opened, Trojan horses act much differently than you expect. Some merely annoy, sending emails to everyone in your address book. Others do serious damage, to the point of stealing passwords and data files. Unlike viruses, Trojan horses are not self-replicating.

Active Trojan horses are an advanced type of Trojan horse. They use unprotected ports to open lines of communication with your computer, and they can ultimately give hackers control over your machine. Active Trojan horses are also called Remote Access Trojans.

What Is A Worm?

A worm is a self-replicating virus that does not alter files but resides in active memory and duplicates itself. Worms use parts of an operating system that are automatic and usually invisible to the user. It is common for worms to be noticed only when their uncontrolled replication consumes system resources, slowing or halting other tasks..