Definitions of commonly used Malware terms (adapted from Wikipedia articles):
Adware: software with advertising functions integrated into or bundled with a program.
Antivirus: software that attempts to identify, neutralize or eliminate malicious software.
Backdoor: a hidden method for bypassing normal computer authentication systems.
Downloader: software that downloads and runs another software, usually a Malware.
Dropper: software that installs a Malware without being infectious itself.
Malware: any malicious software, eg: viruses, trojan horses, worms, etc.
Rootkit: a program (or combination of several programs) designed to take fundamental control (in Unix terms “root” access, in Windows terms “Administrator” access) of a computer system, without authorization by the system’s owners and legitimate managers.
SPAM: unsolicited junk e-mail.
Spamtrap: an e-mail address that is created not for communication, but rather to lure spam. In order to prevent legitimate email from being invited, the e-mail address will typically only be published in a location hidden from view such that an automated e-mail address harvester (used by spammers) can find the email address, but no sender would be encouraged to send messages to the email address for any legitimate purpose.
Spyware: software that is installed surreptitiously on a computer to intercept or take partial control over the user’s interaction with the computer, without the user’s informed consent.
Trojan: software which appears to perform a certain action but in fact performs another. Contrary to popular belief, this action, usually encoded in a hidden payload, may or may not be acutely malicious, but Trojan horses are notorious today for their use in the installation of backdoor programs.
Virus: computer program that can copy itself and infect a computer without permission or knowledge of the user. However, the term “virus” is commonly used, albeit erroneously, to refer to many different types of malware programs. The original virus may modify the copies, or the copies may modify themselves, as occurs in a metamorphic virus.
Worm: malicious programs that copy themselves from system to system, rather than infiltrating legitimate files.
Update 4-28-11: I plan to include a list of familiar names to go with the definition type in the near future as some malware names are not easily distinguishable