Note: I gleaned a lot from Rom Sudo's Gear Faq (I recommend looking at Rom's spreadsheet which shows best items & abilities as well).
Here's my take on Gear:
Acquire gear by buying packs or placing well at the end of an online combat Season. (Rom Sudo says you can get gear in online combat, but I never have.)
One piece of gear can only go to 1 character, even if you have 3 copies of it. Extra copies should be shattered for Shards.
1-3 stars = rarity, best gear is 3 stars (apparently you can evolve 3 star gears to 4 stars). 1& 2 star gears have 2 abilities, but only 1 starts unlocked. To access 2nd ability, you have to level up gear 10x, then Evolve it. This costs a lot of $$ and Shards.
3 star gears are generally strong, and they have a 3rd bonus ability only for one specific character, and they are the only source of Purple Shards (required to Evolve gear) when shattered.
Gear abilities stack, ie 2x +5% block gears = +10%.
Gear abilities vary: boosts to Basic attacks, bonuses to blocking, bonuses to Special attacks. Evolved abilities seem stronger - bonuses chances to Crit, chance to Heal, chance to Life Drain, chances to Cancel or Reverse enemy Special Attacks.
I wrote this up because there's a 1/2 price sale on Gear Packs this weekend (end of Aug 2014) and I wanted to be clear about the mechanics of it myself.
Skip gear if you have no Gold characters. Golds are the gamechangers, Gear is not. If you have a few Gold characters, maybe get some Gear for curiousity, but stil the best buy is the $75k gold pack or gather $400k for Red Son pack. (I haven't been patient enough to do that yet.)
Gear will *NOT* decide a battle in favor of significantly weaker characters, but it could help a lot for evenly matched characters.