A skill is an ability, that may have combat or other effects, most things that can be categorized as skills can be added to the Hotkey bar. Items may also double as skills, as some items have skill buffs that can be used even when the item is not being worn or used.
See Faith Skills
See Emote Skills
See Class Skills
See Zodiac Skills
See Combat Pet Skills
See Items Skills
See Equipment Skills
See Soul Mate Skills