Work is the mode where ChatGPT does the job, not just answers. You describe an outcome, it makes a plan, reads what it needs, works through many steps, asks you to approve important actions, and delivers a finished file.
On web and mobile, Work runs in the cloud and can't reach files on your computer. In the desktop app it can work in a local folder you open for it. Grant access only to the files a task needs. Work uses more of your allowance than Chat, and can be started on a schedule or by an event with a scheduled task.