Many still use this term to refer to a variety of online RPGs, but for the most part, a MUD is completey text-based. Usually the movement of one's character is bound by 'rooms,' or quadrants of area which determine where you are and what you can interact with. Movement generally uses abbreviations for the cardinal directions (n,s,e,w) and sometimes 'in,' 'out,' 'up,' 'down,' etc.
While macros, keybindings and aliases can be created, from the get-go, most commands are typed in, i.e. STAB RAT or BUY SWORD.
MUDs are mainly critisized for being overtly "nerdy" or requiring too much imagination and typing skill. Usually there critisisms come from nearly illiterate RuneScape junkies or snobbish WoW addicts.
Achaea is the best MUD ever.