Multi-User Dungeons, A.K.A. MUD's, used to be more common than they are today. A MUD is a fully, or at least mostly, texted based game. Think of it as an ongoing, interactive work of fiction.
Some people new to MUD's are probably thinking, "You mean like Twitter?"
Uh, ... well, ... minus the 140 character limitation, ... kind of.
MUD servers are commonly in the high fantasy genre, but don't have to be. They can be on any topic ranging from sci-fi, spy vs spy, romance, and even Sims style games. These days, most people play massive multi-player, 3D games. But it is quite relaxing to sit back and figure out a riddle, or slay dragons in a completely text based environment with no worries about the dexterity of your thumbs or mouse-hand. To quote CoffeeMud's website, "a MUD can be the most addictive, rewarding leisure experience of your life, if you give it a chance. There are also numerous web sites and hundreds if not thousands of MUDs to play, each with their own special characteristics and twists to enhance the experience for all concerned."