Ubuntu is a computer Operating System which has grown in popularity and is currently being used by over 20 million people worldwide. It is totally free and distributed as free and easy to use software for PCs. There is also a server version which can be downloaded and used. It is a Linux based desktop operating system available freely. There is both community and professional support avoidable for it. It is based on the principles of Open Source Software and therefore you are encouraged to use it develop it and pass it on, all this free of cost.
The Ubuntu project is supported by Canonical Ltd; a UK based company, owned by South African entrepreneur Mark Shuttleworth and based on the South African philosophy of Ubuntu. The company makes money by providing tech support.
It is free and most importantly easy to use. As you might have already guessed. It was designed to overtake Microsoft Windows as the world's leading desktop operating system. You can download Ubuntu onto a USB drive or CD. You can then run it instead of you current operating system or run it alongside your current operating system. Once you download Ubuntu you will notice that unlike Microsoft where you need to pay for MS Office Ubuntu comes preloaded with basic things like with a word processor (Word), spreadsheet application (Excel), presentation manager ( Powerpoint), PIM client (Outlook), webpage editor (Frontpage), image manipulator ( PhotoShop).
You can try Ubuntu for free now. They also have a page on why you need to use Ubuntu. It is also virus free, which is a huge + point.