What is LangChain and why is it used?

LangChain is a framework for building applications with Large Language Models (LLMs).

It provides:
- **Chains** — link multiple LLM calls together
- **Agents** — let LLMs decide which tools to use
- **Memory** — give LLMs conversation history
- **Retrievers** — connect LLMs to your own data (RAG)

Used for chatbots, document Q&A, data analysis, and AI agents.