The different between Worms, Viruses and Trojan

Viruses

• Viruses are computer programs that are designed to spread themselves from one file to another on a single computer. 
• A virus might rapidly infect every application file on an individual computer, or slowly infect the documents on that computer, but it does not intentionally try to spread itself from that computer to other computers. 
• In most cases, that's where humans come in. 
• We send e-mail document attachments, trade programs on diskettes, or copy files to file servers. 
• When the next unsuspecting user receives the infected file or disk, they spread the virus to their computer, and so on. 

Worms

• Worms, on the other hand, are insidious because they rely less (or not at all) upon human behavior in order to spread themselves from one computer to others. 
• The computer worm is a program that is designed to copy itself from one computer to another over a network (e.g. by using e-mail). 
• The worm spreads itself to many computers over a network, and doesn't wait for a human being to help. 
• This means that computer worms spread much more rapidly than computer viruses.

Trojan

• A Trojan is another type of malware named after the wooden horse the Greeks used to infiltrate Troy. 
• It is a harmful piece of software that looks legal. 
• Users are typically tricked into loading and executing it on their systems. 
• After it is activated, it can achieve any number of attacks on the host, from irritating the user (popping up windows or changing desktops) to damaging the host (deleting files, stealing data, or activating and spreading other malware, such as viruses). 
• Trojans are also known to create back doors to give malicious users access to the system.
• Unlike viruses and worms, Trojans do not reproduce by infecting other files nor do they self-replicate. 
• Trojans must spread through user interaction such as opening an e-mail attachment or downloading and running a file from the Internet.