Malware is a general term for malicious software. Malware can be used to steal information or cause damage to your computer.
Malware includes viruses, worms, Trojan horses, spyware, and rootkits.
Viruses are programs that attach themselves to legitimate programs in order to spread themselves. They can also replicate themselves by attaching copies of themselves onto other files or even CDs. Viruses often cause damage by deleting files or corrupting data on your computer's hard drive. Viruses are often spread through e-mail attachments or downloads from the Internet.
Worms are similar to viruses but have a self-replicating capability that allows them to spread without requiring user intervention. Worms typically infect computers through vulnerabilities in network protocols such as Internet Relay Chat (IRC) or File Transfer Protocol (FTP).
Worms tend not to be as destructive as viruses because they don't require human interaction for infection. However, worms can be extremely damaging if they exploit security holes in applications that are commonly used by many people or businesses, such as Microsoft Office Word or Excel documents.