How Trojans Differ from Viruses and Worms?

Trojan horses differ from computer viruses and worms because they do not replicate. A computer virus is a program that replicates by infecting programs and requires some form of user action to propagate. A worm, however, may or may not infect programs and often does not require any explicit user intervention to replicate. Worms typically replicate and spread much faster than computer viruses.

Trojan horse programs can be roughly divided into those that are deployed by modifying source code and those that are deployed by manually infecting the host executable in much the same way that an executable is infected with a virus. The former deployment method assumes that the Trojan author has the luxury of modifying the original source code to contain the Trojan horse program and that the Trojan author can then compile and deploy the apparently innocent program. This option is not always possible, and so malware authors sometimes resort to modifying preexisting binary executables. The programs that are modified in this way are typically popular programs that are subsequently made available for download or operating system programs that reside on a machine that is under attack.