Glossary
Spyware
The term "spyware" is used to describe all kinds of programs that can collect information about the user's computer-related activities, which information is usually considered personal. This can range from the user's activities on-line (sites visited, searches performed, etc.), to collecting all keypresses made on the keyboard, collecting information about the devices connected to the computer, address books, phone identification numbers (IMEI, if the spyware is installed on a smartphone), conversations the user has participated in using various instant messengers, taking snapshots of the computer screen and even turning on the webcam and microphone connected to the computer and spying on the user's physical activity.
Stealth
Stealth is the ability of a program (usually a virus) to intercept all requests to examine its presence and to modify their results in such a way, that it would look to those issuing them that the program is not present. Depending on the abilities of the programming environment and of the skills of the programmer, different levels of stealth can be achieved. For instance, a virus could try to hide only the fact that the files, it has infected, have increased in size. Or it could intercept every possible attempt to access these files and modify the results of these accesses in such a way that the file look clean. Or, if the virus is the only thing present in a file (or process), it could modify the requests that enumerate the present files (or processes) and entirely hide the presence of the file (or process) containing it.
Trojan Horse
A Trojan horse is a program that does something undocumented that the programmer intended, but that some of its users would not approve of if they knew about it. Unlike the term "computer virus", the term "Trojan horse" cannot be defined formally, because it includes subjective terms like "intent", "harm", "user approval" and so on.
Different people have different intents, what one person considers harmful another might not, and the different people would approve different kinds of actions on their computer. For instance, a program that records keystrokes and sends them to a remote location would be "harmful" for the user whose password is stolen this way but it would be "beneficial" for a law enforcement agency trying to get access to a criminal's computer.
Zombie
The term "zombie" is usually used to indicate a computer which is entirely under the control of an attacker and performs actions, commanded by the latter, against the will (and usually without the knowledge) of its legitimate owner. A large set of zombies controlled by the same attacker (or group of attackers) and usually used for one and the same purpose are called a bot net.
