Ubuntu is certainly an alternative to Windows 8, in that many people use Ubuntu for the same things they would otherwise use Windows for.
You can work with office documents, do web surfing, watch videos and listen to music, access your e-mail, edit videos and photos from your camera... most things you can think of can be accomplished on Ubuntu. And it's a lot nicer to use than any Windows.
However, it's not a drop-in replacement for Windows. You can't run Windows programs. Okay, some can be made to run in a special compatibility layer, but don't come to Ubuntu expecting to run all the same programs and games you used to use. Popular open-source programs from Windows are generally available on Ubuntu, though; like Firefox, Google Chrome, VLC, Scribus, The Gimp.
Ubuntu is also quite different to Windows in terms of how it runs and how you accomplish certain tasks on it. Ubuntu is not difficult to use, it's just different and you'll just have to learn the differences.
So, the short answer is yes: Ubuntu is an alternative to Windows 8. But it's not "Windows-8-but-with-a-different-desktop". It's not "Windows-but-looks-different". It's Ubuntu, and it stands on its own.