What Is a Large Language Model? A Plain-Language Explainer for AI Beginners

A Large Language Model (LLM) is a program that generates text by predicting the most probable next token — a small chunk of text — at each step, based on patterns learned from vast amounts of training data. Multiple LLMs exist, and while they share this core principle, each differs in capacity, modality, reasoning ability, context window size, cost, and access method. Well-known AI products such as ChatGPT, Claude, and Gemini are distinct from the underlying models that power them, such as GPT-4o or Gemini Flash, and a single product may use models from more than one company. So-called open-weight models make their learned numerical parameters — called weights — publicly available, allowing developers to download, run, and fine-tune them on their own infrastructure. These weights encode everything the model learned during training, functioning similarly to the weighted factors a person unconsciously considers when making a repeated decision, like choosing a commute route.