It is a tabletop game that uses dice to introduce an element of chance into the gameplay. Several players portray a character taken from one of D&D's species, such as elves and humans, and take these characters on adventures (or 'campaigns') set by a special player called the Dungeon Master. A Dungeon Master does not play a character but instead organises the campaign setting and tells the story.
D&D set the standard for tabletop RPGs. It is still very popular.