The IPython Notebook is part of the IPython interactive computing environment[1]. You must to install IPython locally(or in the cloud) and run the notebook web server to load and execute the notebook's code live in the browser. These are static HTML representations of the notebooks, think of them as previews, but you can download the JSON .ipynb file and load it up in your IPython Notebook environment and execute the code.

The benefit of the IPython Notebook is sharable JSON file and interactive REPL-like environment that blends code, results, and narrative prose.

[1] http://ipython.org/ipython-doc/stable/interactive/notebook.h...

edit: I should have refreshed the page before I wrote this redundant reply.