You get snow every hour, amount depends on the level you are in. With that snow you can place snow things at your baseplate. It doesn't matter where you place them, for every snowball it costs you'll get 1 HP. You can also attack people with snow balls. You fill in their names(or let it be random)and choose what you want to throw at them. For everything you throw you'll get XP. With XP you can get at higher levels and unlock more snowballs per hour, more parts to build with and more methods of attack(ie you'll unlock icycles when you're at 15)When someone throws something at you you'll get xp too. Your HP will drop though. You can't lose by getting your HP to zero, it's just that when you don't have any hp left people won't be able to throw at you and thus you won't get more xp, except by throwing yourself. You can repair your fort for one snowball, no matter how many HP it lost. Last thing to keep in mind is that after three hours no activity the amount of snow balls you get per hour will halve.
I changed the site theme to St. Patrick's.. Its under Account Settings, look under Site Appearance and its in the dropdown for Site Skin..