Many people use these terms interchangeably but “there actually is a difference between a Snap Cap and a Dummy Round”. Both of these are made to load into your firearm in place of real ammunition and neither will fire, but they have different purposes.
Snap Caps are shaped like a standard cartridge or shotshell but contains no functional components. They are used to to ensure that dry-firing firearms does not cause stress or impact damage to the firing pin and/or the barrel breech.
Some Snap Caps contain a false primer that is either spring-buffered, made of rubber or soft polymer, or none at all. The springs or plastic will absorb the impact force from the firing pin during dry-firing, allowing the user to practice trigger pull or safely test the function of the firearm action without damaging its components.
Dummy Rounds although similar in that it resembles a standard cartridge and has no functional components is really designed for function testing your firearm. They load into the chamber through the magazine like a regular live round but they don’t eject by themselves so you need to manually rack the slide to extract them.
Since both these are inert (they will not fire) they are usually fine for dry-fire practice but plastic snap caps will wear out quickly and should be discarded.
A Word About 22LR
Unlike Center-Fire ammunition like 9mm, 380ACP, 40S&W, 38 Special, etc..
22LR is Rimfire meaning the firing pin or hammer hits the RIM of the cartridge. If the gun is empty this can damage your gun’s firing mechanism, so a snap cap is needed;
A Great use for Snap Caps is for what’s called the “Ball and Dummy Drill” which when done properly will help you minimize “flinching” which is small movement shooters make when pulling the trigger. (but more on that later)