Software that displays advertising banners on Web browsers. While not always threatening, these programs create pop-ups and can cause sluggishness in network connectivity.

Programs that are undesirable but less serious or troublesome as malware.  Grayware includes some spyware, adware, and joke programs.

From Mal

; malware refers to software that is meant to infiltrate or damage a computer system without the owner’s consent and, in many cases, knowledge. Malware programs include computer viruses, rootkits, spyware, and trojan horses.

A malware program that is designed to take full control of the machine’s operating system.

A diagnostic mode used by a computer operating system.  In safe mode, an operating system will have reduced functionality, but isolating problems is much easier.

Computer software that is installed (unknowingly) on a personal computer that is meant to intercept information and/or take partial control over a user’s interaction with the machine. Most spyware programs record various personal information, such as surfing habits, and actively redirect Web Browsers or install 3rd party software without permissi [..]

A program that appears as a legitimate and desirable program, but in actuality, performs undisclosed malicious functions on the machine. Most computer worms are trojan horses to hide the fact that access has been opened to intruders.

A malware computer program that can copy itself and infect a computer without permission or knowledge of the user. Some viruses damage computers by damaging files.  Others open connections for people to take over the machine.

A self-replicating program that uses a network to send copies of itself to other machines, often without any user intervention. At times, the only harm a worm causes is lowered bandwith as it spreads on the network.  However, one of the most common uses of a worm is to install a backdoor on a computer for someone to gain access.