A large language model is an AI system trained on vast amounts of text data that can understand and generate human language. The most well-known LLMs are GPT-4 (OpenAI), Claude (Anthropic), Gemini (Google), and Llama (Meta). They work by predicting the most likely next word in a sequence, but the scale of their training — often on trillions of words — gives them emergent capabilities like reasoning, coding, analysis, and creative writing. They are the technology behind most modern AI assistants and chatbots.