Scikit-learn is a widely-used Python library for machine learning, built on top of NumPy, SciPy, and matplotlib. It provides a comprehensive suite of tools for data preprocessing, model selection, and evaluation, covering a wide range of algorithms such as regression, classification, clustering, and dimensionality reduction. Scikit-learn's simple and consistent API makes it easy to implement machine learning models, from basic to advanced applications. It also integrates with other libraries like pandas for data manipulation and Matplotlib for visualization. Scikit-learn is one of the most popular choices for machine learning in Python due to its ease of use and extensive documentation.