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.
Person 1: Hey, I recently heard DC Comics is rebooting the DCU. It's now going to have a multiverse with 374 universes, Wonder Woman is now a Martian and the Flash is a talking bear! It's called Ultimate Final Last Crisis 2.0!
Person 2: Ugh. I love DC. It has great characters. But they have too many big retcons. I wish they'd just be stricter on maintaining continuity. Is it any wonder why so many people who want to get into comics feel intimidated and don't know where to start?