Core answer
Start here. This is the direct spoken answer to practice first.
Overview
Both can reduce perceived pressure during rapid input, but they control different work and have different timing guarantees.
useDeferredValue lets an expensive part of the UI render a lagging value in the background while the input updates immediately. Debouncing waits for a quiet time before running a callback, often to reduce requests or other repeated work. A deferred value has no fixed delay and does not prevent a request from being made by code that already runs for every input.