NLTK (Natural Language Toolkit) is a comprehensive library for working with human language data (text) in Python. It provides tools for text processing tasks such as tokenization, stemming, lemmatization, and part-of-speech tagging. NLTK also includes a wide range of corpora and lexical resources, making it useful for tasks like text classification, sentiment analysis, and language modeling. It is an essential tool for researchers and developers working in natural language processing (NLP). Despite newer libraries like SpaCy, NLTK remains popular due to its extensive documentation and flexibility.