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Adware:_

Any software application in which advertising banners are displayed while the program is running. The authors of these applications include additional code that delivers the ads, which can be viewed through pop-up windows or through a bar that appears on a computer screen. The justification for adware is that it helps recover programming development cost and helps to hold down the cost for the user.
Adware has been criticized because it usually includes code that tracks a user's personal information and passes it on to third parties, without the user's authorization or knowledge. This practice has been dubbed spyware and has prompted an outcry from computer security and privacy
advocates, including the Electronic Privacy Information Center.

Trojan Horse:-

A destructive program that masquerades as a benign application. Unlike viruses, Trojan horses do not replicate themselves but they can be just as destructive. One of the most insidious types of Trojan horse is a program that claims to rid your computer of viruses but instead introduces viruses onto your computer.
The term comes from the a Greek story of the Trojan War, in which the Greeks give a giant wooden horse to their foes, the Trojans, ostensibly as a peace offering. But after the Trojans drag the horse inside their city walls, Greek soldiers sneak out of the horse's hollow belly and open the city gates, allowing their compatriots to pour in and capture Troy.

Computer Worm:-

A computer worm is a self-replicating computer program, similar to a computer virus. A virus attaches itself to, and becomes part of, another executable program; a worm is self-contained and does not need to be part of another program to propagate itself.
The name 'worm' was taken from The Shockwave Rider, a 1970s science fiction novel by John Brunner. Researchers writing an early paper on experiments in distributed computing noted the similarities between their software and the program described by Brunner and adopted the name.

The first worm to attract wide attention, the Morris worm, was written by Robert Tappan Morris, Jr. at the MIT Artificial intelligence Laboratory. It was released on November 2, 1988, and quickly infected a great many computers on the Internet at the time. It propagated through a number of bugs in BSD Unix and its derivatives. Morris himself was convicted under the US Computer Crime and Abuse Act and received 3 years' probation, community service and a fine in excess of ,000.

In addition to replication, a worm may be designed to do any number of things, such as delete files on a host system or send documents via email. More recent worms may be multi-headed and carry other executables as a payload. However, even in the absence of such a payload, a worm can wreak havoc just with the network traffic generated by its reproduction. Mydoom, for example, caused a noticeable worldwide Internet slowdown at the peak of its spread.

A common payload is for a worm to install a backdoor in the infected computer, as was done by Sobig and Mydoom. These backdoors are used by spam senders for sending junk email or to cloak their website's address.
Dec 28 (15 hours ago)
Saleen
Backdoor
A program that allows a remote user to execute commands and tasks on your computer without your permission. These types of programs are typically used to launch attacks on other computers, distribute copyrighted software or media, or hack other computers.