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 copies of itself (as in
companion viruses). It may damage or corrupt data, change data,
or degrade the performance of your system by utilizing
resources such as memory or disk space. Some virus
scanners detect some viruses. No virus scanners
detect all viruses. No virus scanner can protect
against "any and all viruses, known and unknown,
now and forevermore".

Worm:
Made famous by Robert
Morris, Jr. , worms are programs which reproduce
by copying themselves over and over, system to
system, using upresources and sometimes
slowing down the systems. They are selfcontained
and use the networks to spread, in much the same
way virusesuse files to spread. Some people
say the solution to viruses and wormsis to
just not have any files or networks. They are
probably correct.We would include computers.

Logic Bomb:
Code which will trigger a particular
form of 'attack' when a designated condition is met.
For instance, a logic bomb could delete all files on
Dec. 5th. Unlike a virus, a logic bomb does not make
copies of itself.