It is one of my favorite styles. It is basically an ale fermented with lager yeast, or at least that is the simple explanation. Think of a very clean tasting blonde ale with perhaps some slight fruitiness to it. It is very easy to drink, which is why some people love it as a summer beer. I brew mine all malt, but some recipes use wheat as well. You want to ferment it cool, like around 58 to 62 F if you can, which is why I brew mine in early spring when I can still have low 60's for temps in my basement. If I had a temperature controlled fermentation box, I would brew this year round. It is a 4 to 5 percent beer, so it can be quaffed as a session beer.
For US commercial beers, the BJCP lists: Goose Island Summertime, Alaska Summer Ale, Harpoon Summer Beer, New Holland Lucid, Saint Arnold Fancy Lawnmower, Capitol City Capitol Kölsch, Shiner Kölsch
I haven't tried any of those myself.