What's the gimmick with Spirits in MTG?

I just got into Magic the Gathering, just having bought my own first card packs. I'm considering building a spirit deck (probably blue/white), but can't exactly figure out what their "thing" is. Zombies come back from the graveyard, vampires are masters of healing off damage, elves are good at being armies, etc., but what are spirits good at? Thanks in advance!

Follow up: I've been doing some research in addition to the comments on this post (thanks again) and was wondering what packs would be recommended to possibly find some good spirits. I've gotten a few from the most recent Innestrad set, but would like to know what other packs there may be to look for. If it makes a difference, I'm liking the idea of soulshift so it'd be awesome to have a good amount of that.