A mod, addon, or pack are all names for a collection of models that an author releases. These collections can include anything YS Flight related including maps, aircraft, ground objects, weapon skins, dat updates, etc. Usually, a pack is designated and remembered with acronyms for their names, such as CEP for Ctrl's Extra Pack, GAC for General Air Combat, or HQP for Headquarters Pack, to name a few popular ones. Other packs like Lord Flasheart's Pack, YS Major's Pack, and Styk's Map Pack, are known by their author's name.
A pack also usually contains more than one item, otherwise it is generally known as an addon. It can contain all aircraft models, or maps, or it can be a combination of any and all YSF addons made by that author.
At the moment, there are currently more than 30 major packs for YSFlight, and counting.