Both hold water back behind a separate detection system before it ever enters the pipe. Only one of them floods every head at once.
A companion Concept Explainer already covers why preaction systems exist at all— a separate detection system, not just a pressure drop, has to confirm a fire before water is admitted into the piping. Deluge systems share that exact same front-end idea: an independent detection system, not the sprinkler piping itself, is what decides a fire is real. But that's where the resemblance ends. What each system does with the sprinkler heads themselves — after that detection system trips — is close to opposite, and that difference is the entire point of choosing one over the other.