Viruses and worms are well known types of malicious software. Many threats combine elements from different types of malicious software together. These blended threats don’t fit into any one class, so the term malware, short for malicious software, is used as a catch-all term to describe a number of malicious threats, including viruses, worms, and more. Malware presents arguably the largest security threat to computer users. It can be confusing to understand what the difference is between a virus and a Trojan, but these explanations should help:
A virus is a malicious code that replicates itself. New viruses are discovered daily. Some exist simply to replicate themselves. Others do serious damage such as erasing files or even rendering the computer itself useless.
A worm is similar to a virus. They replicate themselves like viruses, but do not alter files like viruses do. The main difference is that worms reside in memory and usually remain unnoticed until the rate of replication reduces system resources to the point that it becomes noticeable.
A Trojan horse got its name from the story of the Trojan horse in Greek legend. It is a malicious program disguised as a normal application. Trojan horse programs do not replicate themselves like a virus, but they can be sent as attachments to a virus.
A rootkit is a set of tools and utilities that a hacker can use to maintain access once they have hacked a system. The rootkit tools allow them to seek out usernames and passwords, launch attacks against remote systems, and conceal their actions by hiding their files and processes and erasing their activity from system logs.
A bot is a type of malware which allows an attacker to gain complete control over the affected computer. Computers that are infected with a bot are generally referred to as zombies.