Here’s Microsoft on why service packs are better than patches (as well as explaining their meanings for: Product family, Product, Version, Service pack, Patch).
They don’t really clarify Service Pack vs Service Release, claiming it’s the same thing, though at one stage it seemed that an SP is cumulative, whereas SRs often require you to install them consecutively to be up-to-date, eg Office 97 SR1, then SR2b.
This theory is broken with Visio 2000 SP2, which requires SR1 before you install it. Helpfully, SP2 is available for download, but SR1 isn’t! Brilliant! I have a vague feeling that vanilla Visio 2000 was never available for retail sale, but it’s certainly found its way into a few enterprises (such as where I work), so some people are bound to need SR1. But no. Obviously it was taking up too much valuable disk space on the Microsoft servers.