The document provides an overview of the Natural Language Toolkit (NLTK). It discusses that NLTK is a Python library for natural language processing that includes corpora, tokenizers, stemmers, part-of-speech taggers, parsers, and other tools. The document outlines the modules in NLTK and their functionality, such as the nltk.corpus module for corpora, nltk.tokenize and nltk.stem for tokenizers and stemmers, and nltk.tag for part-of-speech tagging. It also provides instructions on installing NLTK and downloading its data.