MALWARE (literally malicious software) is any kind of software that performs unwanted and malicious attacks on your computer. This could be a virus, worm, trojan, adware, or any number of other kinds of programs.
VIRUSES are programs concealed within another program that replicate themselves into different parts of the computer.
TROJANS often look innocuous and do not accurately describe their operation during installation. Once they are installed, they perform whatever it is meant to do, such as open the computer for external attack or encrypting the user’s hard drive.
ADWARE is a very common type of program that can hijack web browsers to display pop-up windows and different websites.
SPYWARE is designed to run quietly and collect information about the user to submit back to a server. This could be anything from websites visited, file names on a hard drive, or even usernames and passwords.
PUPs are potentially unwanted programs. Most adware and spyware fall into this category. They can often manifest as unwanted toolbars attached to web browsers.
Most of these can be removed from a computer without losing valuable data. Some will damage the operating system to the point where it is irrecoverable and needs to be reinstalled.