Trojan horse is a program in which malicious or harmful code is contained inside apparently harmless programming or data in such a way that it can get control and do its chosen form of damage, such as ruining the file allocation table on your hard disk. A Trojan horse may be widely redistributed as part of a computer virus.

Also called a trapdoor. An undocumented way of gaining access to a program, online service or an entire computer system. The backdoor is written by the programmer who creates the code for the program. It is often only known by the programmer. A backdoor is a potential security risk.

In a computer, a worm is a self-replicating virus that does not alter files but resides in active memory and duplicates itself. Worms use parts of an operating system that are automatic and usually invisible to the user. It is common for worms to be noticed only when their uncontrolled replication consumes system resources, slowing or halting other tasks.

Downloader virus or Downloader Trojan is a kind of malicious software, its main purpose is to download malware to the user's computer to run. After poisoning the user's computer, will appear to run slow computer and network security software shut down for no reason, various software and Windows undue error, automatically open the Web page, user information is stolen, etc.

A rootkit is a system created to infiltrate a computer system and then conceal its presence once it has infiltrated. Typically, a rootkit is a set of programs that work together to achieve and maintain this access. Once access has been achieved, the rootkit has access to the administrative controls of a computer system.

Constructor is a hazardous Trojan horse program. It can insert additional malware into infected computers.