Spring is an open-source application framework that supports the development of lightweight, modular, and loosely coupled applications. It is based on core Java technologies like JEE and uses an inversion of control container and aspect-oriented programming to enable lightweight transactions and dependency injection. Spring decouples the definition of application components from the way they are assembled, allowing for less rigid frameworks and supporting testability. It makes it easier to develop and test enterprise applications by providing a non-invasive and lightweight infrastructure.