A large language model is a neural network trained on trillions of text tokens that can generate, understand, and manipulate human language with remarkable fluency. GPT-4, Claude, Gemini, and Llama are prominent examples. LLMs power chatbots, code assistants, translation tools, summarization, and creative writing. Their versatility comes from scale: more parameters and more data yield emergent abilities like reasoning and code generation, though they still hallucinate and require careful deployment.