Pets are cosmetic items applied to the character. They summon creatures that follow the character around, but do not participate in combat.
Pets can be upgraded to +15 and then can evolve to higher tier. Also rarity from common to mythic!
Each pet starts at 10 item level and reaches 360 item level at +15 and tier 8.
Some will be available in-game via quests or from drops.
Pets from the store are in no way better than the normal ones from the game! Store pets have more skins and shaders.

Pet attributes available.

Max HP, Max Mana, Health Regeneration, Mana Regeneration, Item Find, More gold from loot, Recovery Effectiveness and Experience.