NLTK, or the Natural Language Toolkit, is a powerful library in Python designed for working with human language data. It provides easy-to-use interfaces for over 50 different corpora and lexical resources, along with libraries for text processing tasks such as tokenization, parsing, and classification. Developed by Steven Bird and Edward Loper, NLTK is widely used in both academic and commercial settings for natural language processing (NLP) projects. It supports various tasks, including sentiment analysis, language modeling, and machine translation, making it a valuable tool for researchers and developers in the field of NLP.