Viruses are computer programs, usually short pieces of code, that can be spread to other computers through downloaded material, email attachments, or redirected web pages. Many are capable of reproduction, making the removal of these annoyances often quite difficult, and even more are introduced daily. The presence of a virus may often be indicated by browser redirects, unwanted pop-ups, sluggish performance, or even system crashes.
Anti-virus software is designed to protect your computer from certain types of attacks. Some work in conjunction with firewalls to prevent your computer from being used to pass infections on to other machines. Any computer that does not have up-to-date anti-virus and firewall protection can fall prey to viruses, worms, Trojans and other threats.
Anti-virus programs should be updated on a regular basis. Commercial programs should be renewed and licensed at least annually, but the virus definition files used by these programs should be scheduled to update at the very least weekly.
A trojan, or trojan horse virus, is malware that is often transferred through what appears to be a legitimate e-mail or update and will produce a variety of effects. Some are simply a nuisance and do mischief like changing your desktop icons or changing other visual features. Other trojans are designed to destroy computer files, folders and programs. Trojans may also create backdoors allowing access to personal and confidential data. Unlike a worm, trojans do not replicate.
A worm is a malicious program, similar to a virus, except for the manner in which it's spread. A worm duplicates itself, unlike a virus which attempts to infect other files.
The most common type of worm is the email worm. Email worms do not infect other files as do viruses, but spread by sending copies of themselves to any email addresses found on the infected system, and in turn, any of the recipient systems that become infected will also spread copies of the worm to any email addresses on their system. Email worms can spread globally within moments by using this simple tactic.
Internet spam is essentially no more than electronic junk mail, typically unsolicited. It is usually designed to offer you some form of merchandise or service at a price, but may also be used to conceal security threats.
There are a variety of ways that your email address can become the target of spam. You may have registered to an online service or newsletter of some sort. Generally, these services require an email address for registration. In most cases, your email address is added to a mail list and used for regularly scheduled mailings. In some cases, it may also be added to a larger listing and sold to commercial advertisers. Email address that are posted on websites for contact purposes can also be extracted through the use of programs called "spiders" that scan millions of web pages daily for addresses.
Spyware is a software designed to monitor user activity for advertising purposes. Spyware is similar to a Trojan horse as it is typically bundled as a hidden component of freeware or shareware programs. Spyware can gather e-mail addresses, passwords, or banking information and transmit information in the background to another party. They have the ability to monitor keystrokes, scan files, install other spyware programs, read cookies, or change the default home page on the Web browser and can lead to system crashes or general system instability.
Adware is a form of spyware that collects information about the user in order to display advertisements in the Web browser. Some applications that contain adware also track your Internet surfing habits in order to serve ads related to you. When the adware is intrusive like this, it becomes something you should avoid for privacy and security reasons.
Grayware (or greyware) is a general term sometimes used as a classification for applications that behave in a manner that is annoying or undesirable, and yet less serious or troublesome than malware. Grayware encompasses spyware, adware, dialers, joke programs, remote access tools, and any other unwelcome files and programs apart from viruses that are designed to harm the performance of computers on your network.