If you're just starting and need to ask questions, it's important that you don't get the wrong answers and as such it's useful to know how the game calls some stuff in its database first.
Abilities: all spells and clickable stuff like Stand your Ground or passives like Regeneration are abilities. When creating/editing one, it is advisable to look for "abil" on the asskit's table launcher or to scavenge a mod that does that. Because it's usually a lot of tables and they are spread around quite a bit.
Ancillaries: items and followers. They link to effects and those sometimes to abilities. If they enable abilities, they can be used as items for custom battle.
Attributes: stuff like Stalk, or Undead. They are individually hardcoded but you can combine them as you like using the two unit_attributes etc tables. The group is finally assigned in land_units.
Contact Effects: stuff like poison. Neither a true ability nor an effect, but they use some of the tables of the former as they originate as a special ability phase. Found on weapons/projectiles or as imbued from abilities.
Effects: anything from +10% melee attack for Poker Knights to building cap effects (+1 for Ushabti) is an effect. They can be pretty ramified depending on what they do, and there's a lot they could be doing, from nothing to enabling abilities. Only used in campaign: +x MA from an ability (Stand your Ground again) is not an effect. Too much that could be said about them in terms of tables.
Skills: you may instictively call "skills" anything you see on a unit but they have nothing to do with units. Each square thingy you see in a character's skill tree in campaign is instead a Skill. Similarly to Abilities, Skills are defined in a whole lot of tables that start with "character_skill" and carry one or more Effects.
Traits: similar to Skills, but they're not part of the skill tree. They're stuff like Hates Greenskins and are more malleable with scripts to different ends.