LangChain is an open-source framework that helps you build AI applications by connecting large language models to external data and tools.

Think of it as plumbing for AI apps. Instead of writing custom code every time you want an LLM to search your docs or call an API, LangChain gives you pre-built components you can snap together.

Builders use it to create chatbots that remember conversation history, agents that can use tools like calculators or web search, and RAG systems that answer questions based on their own documents. It handles the messy parts like prompt templates, memory management, and chaining multiple LLM calls together.

Available for Python and JavaScript. Free and open-source, though you'll pay for the LLM API calls themselves. Popular alternatives include LlamaIndex for document-focused apps and Haystack for production pipelines.