In creative mode you can build anything you want out of blocks. In survival mode there are animals, zombies, and mummies and you try to survive.
You can build anything you want, there is no limit. You can go into the same world as other people and build with them.
I wish that you could customize your character and that you can see people connected to a different wifi.
Additional Notes from AJG:
Allows you to build structures of any sizes. Houses, Tunnels and basically anything else you can imagine in a 3D world.
Other than the educational building aspects (think of Lego blocks), some school districts (at least in North Carolina) are using the full version as a way for children to build items that they plan and design.