A Python library is a collection of pre-written code that allows developers to perform common tasks without having to write code from scratch. These libraries contain functions and methods that simplify programming in the Python language, making it easier to develop applications, analyze data, or create visualizations. There are many popular Python libraries, such as NumPy for numerical computations, Pandas for data manipulation, and Matplotlib for data visualization. By using these libraries, programmers can save time and effort, focusing on solving problems rather than dealing with low-level coding details.