Marvin is an open-source AI tool for building AI-powered software. It introduces the concept of AI functions, which can perform complex tasks like sentiment analysis or entity extraction without relying on source code. Marvin also includes a flexible chatbot that can assist users given specific instructions and role. Its features include the ability to write AI functions without source code, build chatbots, create plugins, store knowledge, and interact using various modes. Marvin is suitable for users who want to integrate AI into their software development process and have full control over their AI capabilities.