A brief explanation:
Ubuntu is a free, open source Linux operating system. It is named after South African ethnic concept meaning "humanity towards others". Based on a quick search, I found out that an estimated 12 million people are using Ubuntu worldwide.
1) it's free! No strings attached. Also, almost all applications (thousands) are free too (I have not found one that you have to pay for yet). For example, it comes pre-installed with Firefox, but I prefer Google Chrome, so I installed it.
2) No viruses! There is no need for ant-virus software, as viruses aren't typically designed to run on Linux. If you download one, it will just stay in your computer and do nothing.
3) It is open sourced! If you want to design your own app, or modify a key part of your system, you can.
4) It is fast! Even on my laptop booting takes only a couple of seconds.
5) It looks pretty! It has a really nice appearance, and almost everything is customizable.
If you are still not convinced, or want more information, go check out their website at www.ubuntu.com