Quick answer: Most-used LLM application framework — chains, RAG, tools, agents.
LangChain is an open-source framework for building applications with large language models (LLMs). Built by LangChain Inc., it provides a standardized interface for connecting LLMs to external data sources, tools, and APIs through composable components called "chains." The framework simplifies complex LLM workflows by handling prompt management, memory, tool integration, and retrieval-augmented generation (RAG) out of the box. LangChain supports multiple LLM providers including OpenAI, Anthropic, and Hugging Face, making it vendor-agnostic. It's available as open-source software with an optional paid tier called LangSmith for monitoring, debugging, and optimizing LLM applications in production.