The Penn Treebank is a linguistic resource that provides annotated text for natural language processing. It includes a large corpus of sentences from various sources, such as news articles and fiction, which are tagged with grammatical structures. This helps researchers and developers understand the syntax and semantics of the English language. Developed at the University of Pennsylvania, the Penn Treebank has been instrumental in training machine learning models for tasks like part-of-speech tagging and parsing. Its structured data allows for better analysis and understanding of language patterns, making it a valuable tool in computational linguistics and artificial intelligence.