Originally Posted by WhatThePhuc
so it's basically like having a computer inside of a computer
Something like that. Essentially, you can use it to have an operating system on top of your other operating system. Virtual Machines can be used for lots of things, like learning how to set up networks when you only have one computer, or running games on a non-native OS (like running windows stuff on linux or vise versa).
You can also use them to test viruses, malware, antiviruses, firewalls, etc.