If you are referring to Microsoft, they use the term service pack to mean something you download and apply to something you already have installed - NT, W2K, Office, etc. A service release refers to the complete product that has the service packs already applied to it. For example, you can purchase a copy of Office 2000 that has SP1 included in it. You can buy a copy of NT 4 that has SPx included in it, etc.