Ubuntu is an open source software platform, but colloquially, when Ubuntu is referred to it's mainly toward the Ubuntu operating system. Ubuntu is based on Debian and uses the same package management system (deb and apt).
So you want to give Ubuntu a try! That's great. First off, let's grab the Ubuntu .iso file that you'll be needing to install the operating system on your system. Note, an .iso file is an image file that we can burn to a USB/CD. Think of it as a snapshot of the Ubuntu Operating System that we'll burn onto some media disk.