What are they?
Dumb and another micro-transaction thing similar to TF2 hats.
Should I care about these?
Not unless you feel the desire to collect complete sets. iirc, you can get small discounts for completing sets, but it isn't worth it imo. You get cards from playing games, but you won't get a full set. In order to complete a set it will require trading with other steam users or buying them (yay useless micro-transactions). A more in depth explanation.
This is where I got all my info, but I just skimmed it, so I might have left a good bit out.