what are sharrows?

I've been to NYC only a few times and only in the winter. It must be a city thing that NYC doesn't get as much snow as you would assume. It is wicked cold and windy but it doesn't look like too bad of a mess. Maybe in the Twin Cities we have antsy plowers that are on the job quickly but I bike commute (bikomute) all year long.

what would it take to make the City a nice place to ride?