Difference B/w virus and a worm

Virus and worms are programs created to spread thru the communication/network structure, contaminating other machines and other networks if possible. Other results depends on the payload charge it has (most viruses and worms are harmful in any degree, but there are some quite inofensive ones too).

One difference between virus and worm is that a virus needs an existing structure to infect and disguise itself (like a boot sector of a HD partition, a floppy boot sector, an executable file, etc.). A worm, on the other way, is just interested into spreading, but it do not copy itself into another existing file/structure - it simply copies its original file to the target computer and changes its name. If your antivirus catches a virus inside a file, the file can be recovered because the virus is an addendum to the original file. If your antivirus catches a worm on a file, it probably cannot be cleaned except by erasing the file, because too often the worm is the only content of such file.

One more difference between viruses and worms is that a worm only infects using 'bogus' files; viruses (depending on its code) can infect disk sectors, boot sectors or valid executable files. But neither virus nor worm infects using data files.

There are also macro viruses; though they infect some kind of data files, they'll only be harmful if the associated macros (which are in any degree a kind of executable code) are allowed to execute when the document is open. In other words, you can freely copy Word or Excel infected files to and from the internet without getting contaminated, if you do not open them.

Trojan horses are another kind of threat. They stand for a kind of program that's supposed to offer some benefits, but in fact what it does is to allow someone from the internet a way to invade the computer when online and take control. The person controlling the PC can copy sensitive data (credit card and bank account #'s, passwords, etc.), install programs (like keyloggers or spyware) and even format the HD.