One of the "big two" comic companies (the other being Marvel Comics). DC is responsible for such cultural figures as Superman, Batman and Robin, Wonder Woman, Flash, Green Lantern and Aquaman, although there are hundreds of heroes and thousands of characters in the DC Universe. They are mostly known for their superhero comics in the "DC Universe or
DCU" although they do have other divisions, including the major imprint Vertigo Comics which focuses on comics aimed more for adults with many standalone series and the (recently closed and as of August 2011 planned to be integrated into the main DCU) Wildstorm, which had many creator owned superhero comics as well as a manga division.
Starting in 1985 with the famous
Crisis On Infinite Earths DC has had many large scale crossovers in their multiverse that retconned large parts of
continuity, reaching a peak in the late 2000's when they had three major
retcon crossovers starting in 2005 and finishing in 2011 (with possibly more to come). Good job on messing up your continuity so much DC.
One funny thing about DC Comics is that it's not the original company name and the "DC" was taken from one of the more popular (and still ongoing) lines called
Detective Comics, which is the comic that introduced Batman. This means technically the full name is Detective Comics Comics.