A pack is a collection of mods that can be added to an existing load of STEP Core or Extended (some packs work with both, some require Extended). Packs expect STEP to be installed. Since the creation and maintenance of packs is a volunteer activity the packs are typically in different degrees of maintenance. Packs are rarely discontinued, but they are not always maintained regularly. You can usually tell be reading the last page or so of the pack thread (in the Packs forum) how recently the pack was maintained. Packs are intended for users with some experience in modding Skyrim, and often experienced users can work with packs that have small to moderate problems.
Pack descriptions don't typically have a lot of material on compatibility with other packs. Usually you can post questions in a pack thread about compatibility with other packs that interest you, and you will often but not always get answers since it depends how familiar users are with the pack. I currently maintain the A Real Explorer's Guide to Skyrim (REGS) pack and at the beginning of the pack description are comments on the purpose of the pack and compatibility with other packs and guides.
There are also guides for Skyrim (see the Modding Guides forum). These are not completely compatible with the STEP guide, and are also aimed at more experienced modders.