This is free operating system that is used by a few people. It is a very user friendly OS and quick to install and get up and running. This is good for people that are on a tight budget or have no budget for purchasing an OS.
Instead of just installing Ubuntu and then having to go through another installation if you don't like it, you have an option to try it before you install it. This is something that windows doesn't offer when you load up the installation CD If you try it and you you like it, you can easily install it onto your hard drive by a link on the desktop. You can also install Ubuntu alongside an existing OS, such as windows. This way you have the best of both worlds and can switch from one OS to the other quickly and easily.
Ubuntu is also a very stable OS and doesn't require the high end, top spec PC to run, all it requires is a basic PC that most people now don't use as it doesn't run the latest version of windows. These machines are currently well know as redundant and used as spares. Now with Ubuntu, you can install it on one of these machines and make use of it instead of it being stored away collecting dust.