An AI that can take actions on its own. Not just answer, but DO.
A chatbot writes you an email. An agent writes the email AND sends it. That's the difference. An agent has tools (send email, search the web, open a file, run code), a goal, and the ability to decide when to use which tool. It runs in a loop: think → act → look at the result → think again. It keeps going until the goal is met or it hits a limit you set.
Agents are the whole point of building on top of models. A model by itself is a parlor trick. A model plus tools plus a loop is a worker. Every useful AI product shipping in 2026 is some flavor of agent.