What is scikit-learn?

Scikit-learn is a free and open source software that helps you tackle supervised and unsupervised machine learning problems. The software is built entirely in Python and utilizes some of the most popular libraries that Python has to offer, namely NumPy and SciPy. 

The main reason why scikit-learn is very popular stems from the fact that most of the world's most popular machine learning algorithms can be implemented quite quickly in a plug and play format once you know what the core pipeline is like. Another reason is that popular algorithms for classification such as logistic regression and support vector machines are written in Cython. Cython is used to give these algorithms C-like performance and thus makes the use of scikit-learn quite efficient in the process.