There are different biomes, where it really looks like a different world. Here's an example:
There are crazy mountains and lush forests. With Mod packs, you can change almost anything you want.
Then, you get to build anything you can imagine. You can create your own world, or model it after something you've seen before.
For instance, there are loads of worlds that people modeled after Game of Thrones.
You can get started building your own world right now!
All you have to do is download Minecraft for free!
This guy on Formspring.me has been giving it away for free as well. And there's a good profile for Minecraft over here.
Of course, you will need to check out the official Minecraft.net website.
You can use Google to get more info on Mods and techniques.
