concept
A chat assistant answers one message at a time. An agent is given a goal and works on it over several steps: it decides what to do next, takes an action such as searching, running code or using another app, looks at the result, and repeats until it is done or needs help.
Under the hood, an agent is usually a model running in a loop, with access to tools and rules about what it may do.
Because agents act rather than only answer, they can save a lot of effort, and they also need clear limits and a person who can check their work.
Example
Asked to tidy a folder, an agent might list the files, decide on better names, rename them, and report what it changed.