Program or file viruses are pieces of viral code that attach themselves to executable programs. Once the infected program is run, the virus is transferred to your system's memory and may replicate itself further. Macro viruses are currently the most commonly found viruses. They infect files run by applications that use macro languages, like Microsoft Word or Excel. The virus looks like a macro in the file, and when the file is opened, the virus can execute commands understood by the application's macro language.
Multipartite viruses have characteristics of both boot sector viruses and file viruses.
They may start out in the boot sector and spread to applications, or vice versa. While not technically viruses, other malicious programs like worms and Trojan horses usually get lumped in there too. They typically have the same type of results as viruses and are written to create havoc on your system or networks, or both. A worm is a program that replicates itself, but does not necessarily infect other programs. Examples of recent worms are Melissa and LoveLetter, both of which caused widespread havoc.