Dungeons & Dragons is a role-playing game originally created by Gary Gygax and Dave Arneson, and published by TSR. It is now published by Wizards of the Coast, a division of Hasbro, and is the most played RPG in the world.
Read more at Wikipedia.
If your question is about a specific version you probably should tag with it instead:
- dungeons-and-dragons is for questions relevant across versions
- odnd - The first D&D, aka 0e, brown box/white box from 1974
- dnd-basic - The first Basic D&D by Holmes from 1977
- dnd-bx - Moldvay Basic and Cook Expert from 1981
- dnd-becmi - Mentzer Red Box + (expert, companion, master, immortal) from 1983
- adnd - Advanced Dungeons & Dragons, aka 1e, from 1977
- adnd-2e - Advanced Dungeons & Dragons Second Edition from 1989
- dnd-3e - Dungeons & Dragons Third Edition from 2000
- dnd-3.5e - Dungeons & Dragons 3.5 Edition from 2003
- dnd-4e - Dungeons & Dragons Fourth Edition from 2008 (includes Essentials)
- dnd-next - D&D Next, aka 5e, in playtest in 2012
- pathfinder - Fork of D&D 3.5e by Paizo in 2008
- Also there are various retroclones of specific editions with their own tags.