Trojan:
Remember the Trojan Horse? Bad guys hid inside it until they could get
into the city to do their evil deed. A trojan computer program is
similar. It is a program which does an unauthorized function, hidden
inside an authorized program. It does something other than what it
claims to do, usually something malicious (although not necessarily!),
and it is intended by the author to do whatever it does. If it's not
intentional, its called a 'bug' or, in some cases, a feature Some
virus scanning programs detect some trojans. Some virus scanning
programs don't detect any trojans. No virus scanners detect all
trojans.
Virus:
A virus is an independent program which reproduces itself. It may
attach to other programs, it may create...