Binder: To use Binder, go to this site https://mybinder.org/ and paste in a copy of your Github repo with your notebooks. Read the notes on the site to figure out how to make this work. For instance, you need a file called requirements.txt that lists the Python packages that are needed to run.
Mybinder will give you something to copy, that looks like this:
The notebook is then "live." You can edit it, make changes, run it. But it only exists ephemerally, which means it will disappear after some time. But, you can always download it directly onto your machine.
Viewer: nbviewer lets you see a rendered Jupyter notebook, but it is not "live." That is, you can't make any changes. To use it, you take the web address http://nbviewer.ipython.org/ and tack onto the end url/ and the address of your notebook. Something like this: