In the German tradition, Kölsch is probably considered a "lager" because it's "lagered" or cold conditioned for an extended period of time, but it's an oddity because it's a top fermenting yeast strain that works best at lower temps (close-to-lager strains). It produces some subdued fruity esters because it's an ale strain.
Kölsch, Alt, California Common, and some would argue even Kellerbier are the "category busting" beers that blur the lines between Ale and Lager and draw distinctions between the BJCP and German categorization schemes.
Kolsch is one of the most unforgiving beer styles, so unless you've mastered everything and have strict temp control and a German-like obsession for process don't try to make it at home to avoid disappointment.
Adam