sklearn (scikit-learn) is a machine learning library for Python. In practice, it's useful for integrating a whole zoo of machine learning models for a range of tasks (supervised, unsupervised) and varying strategies within these domains (e.g., decision-tree based models, regression, neural networks) into a simple API, where the following three lines of Python code do 80% of the ML work for you.
>>> model = [insert sklearn class here]
>>> model.fit(trainX, trainY)
>>> model.predict(testX)