Even though we consider Trojans, backdoors, spyware, key loggers, worms and adware are as viruses, they are actually not viruses. For example Trojan is a program which is supposed to do a specific job, but it will do something else. It comes for a different purpose though. Viruses and these programs put together are called as malware.
In this regards worms are not a virus even though it spreads like a one in e mails and all the stuff. Worms will create duplicate copies of itself everywhere possible where as viruses will infect into other files. Logic bombs on the other hand will be a program made to delete files or something unexpected like this. It will start at a specific time and day as programmed.
Dialers are more dangerous if a modem is used. It will dial some phone numbers without the user`s knowledge and it will give a big bill to the user. These dialers will be downloaded automatically.
Adware is a platform where ads will be published on software and the revenues generated will be used by the developers of the adware. Opera browser is one such adware which holds others advertisements. Risk of this comes when the user unknowingly agrees for the contracts of these advertisements.
Spyware are set of programs which used to send the data from an infected computer to a specific location. This is used to gather browsing patterns for browsers by browser developers. Sometimes they used to extract confidential information like credit card numbers.
Backdoors are used to control the computers remotely by programmers. Worm writer aims to install a backdoor in a system to get access to other programs.
Virus hoaxes are not coded programs. They are the forwarded programs which fill the mail boxes and servers. Some people forward mails to friends without even reading it. These are hoaxes.
Almost all the anti virus software available these days are able to catch almost all these viruses except hoaxes which is not a code. All these are considered as viruses in this article for easy usage. Updates and add ons can almost detect even spyware now days.
Anti virus programs are very important to prevent the computers getting infected from viruses. They are a set of or an individual program. Main part of an anti virus is the screening. It will look through all the possible files and detect the patterns which are matching the patterns of viruses. These patterns are decided on the virus definitions in the anti virus program. After detecting a pattern it will try to remove the pattern from the infected file without damaging other parts of it. If this removing process is not successful, it will be quarantined in a separate folder or will delete to avoid further damage.
Heuristic scanning is a process which used to identify latest virus threats. Latest once will not have definitions in the anti virus program. Hence technically it should not be detected. But this method will identify suspected patterns as viruses and give warnings. Mainly if its affecting windows setup, it will be quarantined immediately. This is a percentage process. It can be wrong at times. If the user sends this report to the anti virus developer they will analyze it and will develop an anti virus for it.