Malware is a generic name given to a kind of software designed for the purpose of harming computer systems, stealing information, or harassing users. Viruses, Trojans and worms can be given as examples of such software.
Computer Viruses
A computer virus is a type of computer program that hides itself in other files and runs without the user's knowledge. The two essential features of the viruses are that they can self-copy and self-run themselves independently of the user's command. Your computer may be infected if it reacts abnormally, messages and pop-up screens appear, changes are made to files, there are disappearances of the files, or if the computer responds slowly or does not respond.
Trojans
A Trojan is a harmful computer program that enters your computer by hiding itself inside another program. Trojans, unlike viruses, are not usually used for copying or damaging computers. Trojans are often used with the intention to obtain, delete or change the data on your computer, access information such as user names and passwords by saving the entries made from the keyboard and share them with third parties from using various channels, disable the security walls and anti-virus programs, or allow other viruses to enter your system.