NestJS is a progressive Node.js framework that combines object-oriented, functional, and reactive programming paradigms to build scalable server-side applications. Built on top of Express or Fastify, it provides a structured architecture using modules, controllers, and providers with built-in dependency injection. The framework offers TypeScript-first development, decorators for routing and validation, middleware support, guards for authorization, exception filters for error handling, and interceptors for response transformation. It includes comprehensive database integration options with TypeORM, Mongoose, and Prisma, along with built-in testing capabilities and CLI tools for code generation.