What are spirit gems?
Spirit Gems are a new collectible which act as a currency for the spirit shop, in the spirit shops you can purchase cosmetics, lives and soul orbs in different quantities, each enemy you defeat will grant you 10 gems and every level has around 100-140 gems excluding enemies so you shouldn't have to grind for hours just for one single trail.
What types of cosmetics are there?
At launch there will only be one type of cosmetic which will be trails you can equip to Kaz but in future major updates there will be more types of cosmetics added, one idea for other cosmetics currently being floated around is Pets which would follow Kaz throughout the levels.
Note: Cosmetics are not reset upon selecting new game and neither is your soul orbs and spirit gems, this is so you can continue to use your rewards and soul orbs as a weird new game plus.
How many cosmetics will there be from launch?
From launch there will be 10 trails for you to purchase with spirit gems from the store, this should require you to back track through levels to obtain all of them but you can still get a decent handful (of your choice) from playing naturally.