Polymer is an open-source JavaScript library developed by Google for building web applications using Web Components. It provides polyfills and a set of custom elements to create reusable, encapsulated HTML elements, enabling developers to build complex applications using a modular approach. The library focuses on leveraging the browser's native capabilities to enhance performance and ease of development. The Polymer Project website offers resources, documentation, and tools designed to help developers create faster, more maintainable web applications.