A virus is an executable
program that installs itself on other computers via emails, downloads,
floppies, and other removable devices, and causes benign or malicious
harm to the host system. They can infect .exe files – programs
– and make them crash. They may display a friendly message and
exit, or crash your computer every minute!
Boot sector virus:
Resides in the boot sector of your hard disk or floppy. Starts with
your computer.
Email virus:
Spreads via email attachments. Can copy your address book and spread
using its addresses. Can even spread without any attachments. Classic
example: Melissa.
Macro virus: Resides
on document files like MS-Word as a small program. Spreads via Word
attachments.
Trojans:
Can not spread like a virus. A harmless looking word processor or music
sampler could be hiding a malicious program inside.
Worms:
Spread via networks using emails and file sharing. They do not infect
other programs, but keep on replicating themselves in order to clog
the network and bring it to a halt. Eg. Morris for ARPANET.
Spyware:
Installs itself on your computer
with your consent – as part of some freeware you have downloaded,
like a Trojan, and then continuously spies on your network activity,
Web sites visited, passwords, cookies, emails, and other information.
This is then passed over to outside parties who use it to launch further
attacks on other systems and popular web sites, such as what is called
‘denial-of-service’ attack. It may also display advertisements
on your screen.