What this one does is add SNOW! and associated cold weather issues as challenge conditions for players to deal with. That'll be things like ensuring adequate heating so keeping up with increased electricity demand or adding water-based heat systems and keeping the roads open.
"New snowplow services will be required to prevent traffic from freezing in place, while new all-weather trams and expanded public transit options can help residents and visitors alike get around town – and visit the new winter parks and landmarks available in Snowfall."
What I'm hearing in my head now is this:
There's also the addition of things like cosmetic rain and fog for existing maps, trams and so on. I'll put the full list from the announcement here because I like all the puns except World Warmth, Too:
Now is the Winter of This Content: Rain and fog for existing maps, plus a new “Winter” map theme with snow and all of its challenges
The Streetcars You Desire: Expanded public transportation options, including easier management of existing lines – and Trams, a new system demanded by fans
World Warmth, Too: Cold weather increases demands for electricity, unless new water-based heat systems are in place
Plowers to the People: Build a snowplow depot to keep traffic flowing during winter weather, and employ new road maintenance systems to keep your streets in shape
New Chirps: #yeahthereare
Actually, that rain and fog point seems to also be part of an upcoming free update so it looks like you'll get the cosmetic weather regardless.
Cities: Skylines - Snowfall is apparently Coming Soon.