Notebook.community is an effort to archive the most interesting open-source Jupyter notebooks.
The Jupyter Notebook is an open-source web application that allows you to create and share documents that contain live code, equations, visualizations and narrative text. Uses include: data cleaning and transformation, numerical simulation, statistical modeling, data visualization, machine learning, and much more. 
Jupyter notebook is a free open-source software for everyone to use. It is developed by Project Jupyter, which "exists to develop open-source software, open-standards, and services for interactive computing across dozens of programming languages." 
Jupyter notebooks usually aren't public by default, and publishing them takes a little effort. There are several ways to share notebooks:
Many of these platforms also allow granting access to specific people rather then general public.
Jupyter notebooks are often used for doing exploratory programming - quickly iterating on code, where the end goal evolves as new insights come up, and for presenting work which interleaves text and code. Jupyter notebooks are heavily used in data science and related disciplines.