This article provides a detailed introduction to LangExtract, a Python library specifically designed for leveraging Large Language Models (LLMs) to extract structured information from unstructured text. It delves into LangExtract's core capabilities, including precise source text mapping, reliable structured output, optimized long document processing, interactive visualization, and flexible support for various LLM models across arbitrary domains. The article then demonstrates, with code examples, how to define extraction tasks, provide examples, run extraction, and visualize the results. Special emphasis is placed on utilizing LLM's world knowledge to enhance extraction quality and optimized methods for handling long documents. Furthermore, it offers installation guides (including PyPI, source code, Docker), detailed API key setup methods, and instructions on integrating OpenAI models and local Ollama models. Finally, through multiple practical application cases such as full-text extraction from 'Romeo and Juliet', drug extraction, and radiology report structuring, the article showcases LangExtract's powerful functionalities and broad applicability.