What's the difference between Scheduling and Deferring a task?

A task with a future Start Date is scheduled. A task with a Due Date can be deferred by moving that due date forward.

Schedule — set when the task should begin. Useful for work you do not want to see yet.
Defer — push out the due date. Useful for postponing a deadline or something that is about to slip.

Examples:

Schedule Prepare quarterly report to start on Monday and be due on Friday. It stays out of the way until Monday, then appears during the working window.
Defer Pay invoice from today to Friday. It remains a due-date task, but the due date moves to Friday.

tip

Use Start Date for "do not show this until then." Use Due Date for "this is when it must be done."