There are different kinds of malicious software like virus, Viruses, Worms, Trojans, Logic bombs, Spyware, Adware, Grayware, Hoaxes
A virus is a piece of code that spreads from one computer to another by attaching itself to other files. The code in a virus corrupts and erases files on a user's computer, or performs other destructive or annoying tasks, when the file to which it was attached is opened or executed. Viruses and other types of malicious code are frequently introduced via email attachments. The term virus is often used as an umbrella term to refer to many types of malicious software.
A worm is a piece of code that spreads from one computer to another on its own, not by attaching itself to another file. Like a virus, a worm can corrupt or erase files on your hard drive or perform other destructive or annoying operations.
A Trojans, or Trojan horse, is malicious code that masquerades as a harmless file. When a user executes the Trojan, thinking it is a legitimate application, it can perform damaging or annoying operations. Or, it can continue to masquerade as a legitimate program, while in the background it captures input from the user and transmits the data to an attacker.
A logic bomb is a piece of code that sits dormant on a user's computer until it is triggered by a specific event, such as a specific date. Once the code is trig¬gered, the logic bomb "detonates," erasing and corrupting data on the user's computer or performing other destructive or annoying operations.
Spyware is unwanted software that runs in the background to monitor system activities and send collected personal user data to a third party.
Adware is unwanted software loaded onto a system for the purposes of present¬ing commercial advertisements to the user. The adware can run in the background and collect data about the user in order to present customized advertisements. The adware can also function as spyware. The user is often unaware that the adware has penetrated the system. The adware itself can create annoyances or adverse system conditions, or the adware can be a vector for introducing other types of malicious software.
Grayware is a general classification for any unwanted software that produces harmful or annoying effects. A joke program that makes the computer screen image vibrates or a drive door open or shut would be a form of Grayware.
In the electronic world, spam originally referred to frequent, repetitive, and excessive postings in electronic bulletin boards. It is now generally taken to mean unsolicited or distasteful commercial email from anonymous sources. Spam creates nuisance conditions by filling user mailboxes with unwanted or inappropriate messages, and impedes email system performance by filling up email servers' storage areas and generating excessive network traffic. Although not technically software, spam travels via email software, which is why it is often considered to be a type of malicious software.
A hoax is any message containing incorrect or misleading information that is disseminated to multiple users through unofficial channels. Hoaxes do not have to be electronic, although electronic distribution systems facilitate their propagation. Although not technically software, some hoaxes travel via email software, which is why they are often considered to be a type of malicious software.
Hoaxes can be relatively benign, such as an email letter soliciting "get well" cards for a fictitious ill child. In this case, the main threat is from users who widely disseminate the hoax email, clogging communications systems and annoying other recipients.
However, hoaxes often improperly alert users to the existence of unsubstanti¬ated virus threats. Rather than validating the information, users often react by following instructions in the hoax that might cause system damage or introduce further malicious software.