What is the diffrence between a computer virus and a worm
Is there anything more similar to them?
A worm is a kind of virus. It spreads through the Internet.
By definition, a virus attaches itself to an existing program and becomes part of its code (there are variations on the theme like boot sector viruses). It spreads from program to program (or disk to disk, in the case of boot sector infectors). A virus is not a self-contained program, it is a code fragment that "parasitises" a host program or reserved disk region.
A worm is a self-contained replicating program that can send itself autonomously out over a network. It does not depend on other executable programs for its spread.
At least, this is the way I understand it. There are other entities like trojan horses and logic bombs. Neither of these entities is self-replicating (although certain virii and worms have trojan-like code) but all 4 entities come under the heading of "malicious code".
There's a comprehensive and also entertaining free e-book by Mark Ludwig on malware, http://www.iirg.net/The_little_black_of_computer_virus.pdf [Broken], "The little black book of computer viruses".
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