Malware

Malware, short for malicious software, is software designed to infiltrate a computer without the owner's informed consent. The expression is a general term used by computer professionals to mean a variety of forms of hostile, intrusive, or annoying software or program code.Software is considered malware based on the perceived intent of the creator rather than any particular features. Malware includes computer viruses, worms, trojan horses, most rootkits, spyware, dishonest adware, crimeware and other malicious and unwanted software. 

Virus

A malicious self-replicating piece of code which inserts itself into other files.  Viruses corrupt or devour files on a targeted computer.

AntiVirus

A piece of software designed to prevent infection of the operating system by a virus.

Trojan Horse

A Trojan horse, or Trojan for short, is a term used to describe malware that appears, to the user, to perform a desirable function but, in fact, facilitates unauthorized access to the user's computer system. The term comes from the Trojan horse story in Greek mythology. Trojan horses are not self-replicating which distinguishes them from viruses and worms. Additionally, they require interaction with a hacker to fulfil their purpose. The hacker need not be the individual responsible for distributing the Trojan horse. It is possible for hackers to scan computers on a network using a port scanner in the hope of finding one with a Trojan horse installed.

Worm

A computer worm is a self-replicating computer program.  It uses a network to send copies of itself to other nodes (computers on the network) and it may do so without any user intervention.  Unlike a virus, it does not need to attach itself to an existing program.  Worms almost always cause at least some harm to the network, if only by consuming bandwidth, whereas viruses almost always corrupt or devour files on a targeted computer.

Rootkit

A rootkit is a software system that consists of one or more programs designed to obscure the fact that a system has been compromised. Contrary to what its name may imply, a rootkit does not grant a user administrator privileges, as it requires prior access to execute and tamper with system files and processes. An attacker may use a rootkit to replace vital system executables, which may then be used to hide processes and files the attacker has installed, along with the presence of the rootkit. Access to the hardware, e.g., the reset switch, is rarely required, as a rootkit is intended to seize control of the operating system. Typically, rootkits act to obscure their presence on the system through subversion or evasion of standard operating system security scan and surveillance mechanisms such as anti-virus or anti-spyware scan. Often, they are Trojans as well, thus fooling users into believing they are safe to run on their systems. Techniques used to accomplish this can include concealing running processes from monitoring programs, or hiding files or system data from the operating system.  Rootkits may also install a "back door" in a system by replacing the login mechanism with an executable that accepts a secret login combination, which, in turn, allows an attacker to access the system, regardless of the changes to the actual accounts on the system.

Spyware

Spyware is a type of malware that is installed on computers and that collects information about users without their knowledge. The presence of spyware is typically hidden from the user. Typically, spyware is secretly installed on the user's personal computer. Sometimes, however, spywares such as keyloggers are installed by the owner of a shared, corporate, or public computer on purpose in order to secretly monitor other users.

Keylogger

A keylogger is usually a small program that monitors each keystroke a user types typically in a covert manner so that the person using the keyboard is unaware that their actions are being monitored.  Keyloggers are often Trojans (programs that pretend to do one thing and actually have hidden features such as the keylogging code).

It can also be a hardware device (right).  As a hardware device, a keylogger is a small battery-sized plug that serves as a connector between the user's keyboard and computer. Because the device resembles an ordinary keyboard plug, it is relatively easy for someone who wants to monitor a user's behaviour to physically hide such a device "in plain sight." (It also helps that most workstation keyboards plug into the back of the computer.) As the user types, the device collects each keystroke and saves it as text in its own miniature hard drive. At a later point in time, the person who installed the keylogger must return and physically remove the device in order to access the information the device has gathered.