LangChain is a comprehensive framework for building applications powered by large language models (LLMs), transforming simple prompts into production-ready AI agents. Originally released in 2022, it has evolved into a complete ecosystem including LangChain (the framework), LangGraph (stateful workflows), and LangSmith (observability/debugging). LangChain abstracts the complexity of chaining LLM calls, managing memory, integrating tools, and orchestrating retrieval-augmented generation (RAG) pipelines—letting developers focus on application logic rather than infrastructure. A critical mental model: every component implements the Runnable interface (invoke, stream, batch), enabling composability and standardized execution patterns across chains, agents, and retrievers.