One simple objective, surprising range
An LLM is trained on a deceptively narrow task: given some text, predict what comes next. Do that across a huge slice of human writing, with enough parameters, and the model absorbs grammar, facts, styles and a good deal of reasoning as a side effect. The result is a single generative AI model that can be pointed at translation, drafting, coding or question-answering without being retrained for each.
Almost all of them are built on the Transformer architecture, the design that made training at this scale practical. You steer them at run time through prompt engineering, and you keep them factual by feeding in trusted context with retrieval-augmented generation.
Why "foundation" model
Because one pre-trained model serves as a base for many downstream uses — adapted by prompting or light fine-tuning rather than trained from scratch each time.
