, keyloggers and logic bombs are not by definition, viruses. Along with viruses as a group they are referred to as malware. Trojans for example, are programs that purport to do a certain function but in reality do another, like pretending to be a game but really harvesting all your email addresses and sending them to spammers.
Likewise, worms are self-replicating programs that spread like viruses, but the distinction is that viruses infect other files, whereas worms create complete copies of themselves and spread without a carrier executable. Worms most commonly use vulnerabilities in e-mail programs to distribute themselves widely and quickly. Logic bombs are programs written to do something unexpected - such as deleting all your files - at a triggered event such as a date.
Dialers are software components usually downloaded from websites without the user's knowledge. They use local modems to dial out to costly phone services in order to accrue charges on a user's account.
Not all adware programs are malicious. Adware refers to programs whose development is funded by the advertising revenues generated through ads shown while using the program. The Opera web browser is a good example of legitimate adware. However, some adware programs trick the user into agreeing to the installation of other programs, many of which are spyware.
Spyware programs are designed to capture information from the infected computer and return it to their controller. Much spyware is designed to record browsing patterns for marketing analysis, but some less benign spyware applications harvest credit card numbers, passwords and personal information.
Backdoors are programs designed to provide an attacker with remote control of a computer. They are often found within Trojans and their installation is often also a goal of worm writers.
Virus hoaxes also deserve a mention here. Although virus hoaxes do not use any actual computer code, they are still able to spread confusion and overload mail servers by using language to exploit the good nature of people who pass them on to their friends and colleagues without verifying their content first.
With the exception of hoaxes, all good antivirus programs will detect trojans, backdoors, worms and logic bombs. For the purposes of simplicity, they will be grouped together and referred to as viruses in this guide. However, an increasing number of vendors now either extend their malware capabilities to include adware, spyware and dialers or are bundling third-party products for the purpose. These applications are discussed in the Spyware Buying Guide (see link>.