What is a trojan/worm/virus/logic bomb?

This FAQ answer was written by Theora:

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 up
     resources and sometimes slowing
down the systems. They are self
     contained and use the networks to spread, in much the same way viruses
 
   use files to spread. Some people say the solution to viruses and worms
     is 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.