“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.”
A notebook consists mainly of markdown cells to build narratives around your code and code cells to define computional code respectively show output of executed code (see figure 1).
Jupyter offers the possibily to explain and run your code in blocks, which is quite nice – not only for data scientists, but for tutorial writers as well: you don’t have to do all the copy&paste stuff anymore. It supports code in many programming languages, but Python is a requirement for installing the Jupyter Notebook itself. To get a brief introduction visit this side. The maintainers recommend to use the Anaconda Distribution, which includes Python, the Jupyter Notebook and other commonly used packages.