Stefan Mileschin

Most people seem to call every type of malware a “virus”, but that isn’t technically accurate. You’ve probably heard of many more terms beyond virus: malware, worm, Trojan, rootkit, keylogger, spyware, and more. But what do all these terms mean?

These terms aren’t just used by geeks. They make their way into even mainstream news stories about the latest web security problems and tech scares. Understanding them will help you understand the dangers your\ hear about.
Malware

The word “malware” is short for “malicious software.” Many people use the word “virus” to indicate any type of harmful software, but a virus is actually just a specific type of malware. The word “malware” encompasses all harmful software, including all the ones listed below.
Virus

Let’s start with viruses. A virus is a type of malware that copies itself by infecting other files, just as viruses in the real world infect biological cells and use those biological cells to reproduce copies of themselves.

A virus can do many different things — watch in the background and steal your passwords, display advertisements, or just crash your computer — but the key thing that makes it a virus is how it spreads. When you run a virus, it will infect programs on your computer. When you run the program on another computer, the virus will infect programs on that computer, and so on. For example, a virus might infect program files on a USB stick. When the programs on that USB stick are run on another computer, the virus runs on the other computer and infects more program files. The virus will continue to spread in this way.
Worm

A worm is similar to a virus, but it spreads a different way. Rather than infecting files and relying on human activity to move those files around and run them on different systems, a worm spreads over computer networks on its own accord.

http://www.howtogeek.com/174985/not-...rms-explained/