Spring is an open source framework for building Java applications. It provides features like dependency injection, aspect-oriented programming, and abstraction layers for web services, security, and data access. Spring aims to simplify enterprise application development by reducing boilerplate code and providing a flexible, configurable architecture. It consists of several core modules that can be used independently or together, including support for web applications, data access, transactions, and more. Spring promotes loose coupling between application layers and components through its lightweight container and declarative configuration.