Well, finally something I know about. If you want to know about beer, cooking, or audio, I'm your guy. Anything else I comment on simply dismiss...
There are really two basic types of beer: lagers and ales. Lagers are commonly known by brewers as 'bottom fermented' - they are fermented at relativly cool temperatures for longish periods of time. Ales are commonly referred to as 'top fermented' and are fermented at 'room temperatures', typically 65-75F. Ales also takes less time from wort to drinking, typically somewhere in the neighborhood of 4-5 weeks from initial brewing to enjoying.
That being said, there really aren't any dark lagers. If you want a dark beer, you want an ale - typically a brown or a stout. Browns are really typically considered medium ales (as compared to stouts (dark) and pales (light)), but are relatively well-bodied, often rather sweet as they utilize relatively few hops for flavoring (used to add bitterness - see IPAs). For real flavor in cooking, I recommend looking for a tasty stout - it'll be full in flavor which won't dilute in cooking, even over high heat.
Some brands to look for are Rogue, Dogfish Head, and Stone. Any of those medium/dark ales (browns and stouts) are wonderful to cook with - great flavors that compliment many foods. You may need to look in speciality stores to find those brands - they are unlikely to be found at your local grocery store...
Hope this helps.