A Large Language Model is a Transformer trained on very large amounts of text. It learns to predict the next token given all the previous tokens.
The scale hypothesis
Somewhere around a hundred billion parameters, models like GPT-5, Claude, and Gemini began to look qualitatively different from their smaller siblings.
Consumer models
The most visible expression of all this is ChatGPT, released by OpenAI in late 2022.
The bitter lesson: scale beats cleverness, more often than we like.