Please clear these terms with brief explanation.

Dear, the brief explanation is given below to all these terms.
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Dialler - A program that typically dials a premium rate number that has per minute charges over and above the typical call charge. These calls are with the intent of gaining access to pornographic material.

Hijackers - A program that attempts to hijack certain Internet functions like redirecting your start page to the hijacker's own start page, redirecting search queries to a undesired search engine, or replace search results from popular search engines with their own information.

Spyware - A program that monitors your activity or information on your computer and sends that information to a remote computer without your knowledge.

Trojan - A program that has been designed to appear innocent but has been intentionally designed to cause some malicious activity or to provide a backdoor to your system.

Virus - A program that when run, has the ability to self-replicate by infecting other programs and files on your computer. These programs can have many effects ranging from wiping your hard drive, displaying a joke in a small box, or doing nothing at all except to replicate itself. These types of infections tend to be localized to your computer and not have the ability to spread to another computer on their own. The word virus has incorrectly become a general term that encompasses trojans, worms, and viruses.

Worm - A program that when run, has the ability to spread to other computers on its own using either mass-mailing techniques to email addresses found on your computer or by using the Internet to infect a remote computer using known security holes.

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Hope you have the answer to your question now.

Best Regards.

Hijackers-Like airplane hijackers, for example you want to open www.yahoo.com but your browser is opening another webpage it is called browser hijacker.
Viruse- a program which makes your pc slow,unresponsive etc.
Worn- it is more like a viruse but it acts slowly
Trojan- it is a program pretending to be a useful one but actually it is harmful one.
Spyware- it is a programe who sends information without the knowledge of computer owner