LangChain is an open source framework, for Python or Javascript, designed to simplify the writing of applications using LLMs. LangChain enables developers to design templates for prompts for LLMs, like GPT-4, and fill in those templates with external data, such data that comes from a vector database or feature store. Examples of applications built with LangChain include document analysis and summarization, chatbots, code analysis, and source code generation.