DM's Guild (short for Dungeon Master's Guild) is an online store spun off from DriveThruRPG under the authority of Wizards of the Coast. Like DriveThruRPG, it's an online store dedicated to selling roleplaying game material - but DM's Guild focuses exclusively on content related to Dungeons & Dragons.
Created as a tie-in to Dungeons & Dragons 5th Edition, DM's Guild sells PDFs and printed material, combining an ever-growing library of classic D&D material from every edition from Basic to 4th (which itself is slowly converted into print-on-demand format) with both official 5e sourcebooks and fan-made material for 5th edition. Basically, if you really like your 5e homebrew and think you can make some money from it, the DM's Guild will let you sell it to the public. Or, if you want to get your hands on an older book that won't cost you an arm and a leg like it would on Ebay, DM's Guild will cover you for that.