The Snowballs are a promo where the UFOs have been replaced by snowballs. You can catch them just like UFOs and when you do you'll be taken to the contest page where you can throw the now caught snowball to anyone on your friends list. You get a gift [the same old stuff that's in regular UFOs] and if you throw it to a friend, they'll also get that gift. If you don't throw it you'll lose the second gift.
The top 10 throwers will get a Nyx pack, the next 40 will get a Horn of Plenty, and the next 50 will get a Medusa's Blood.
After that your gifts are the things you won from the snowballs.
As always, happy Howrseing,