Interactive fiction or IF (also known as text adventures) are a genre of computer adventure games where the player types in commands and receives text responses describing their environment and how their actions have affected the IF world.
They have been around since the 1970s and during the 1980s they were commercially popular but have long since fallen out of fashion. In fact, there's nothing funnier than sitting a modern gamer down in front of one and observing their response ('Where's the graphics? It's all text!'). Nowadays there is a dedicated online IF community still producing new games and taking the genre in new directions.
The player types in what they want to do - minimally a verb e.g. JUMP or a verb and noun e.g. THROW AXE. Adjectives can be used to disambiguate between objects e.g. TAKE GREEN KEY, and prepositions allow more complex commands e.g. TIE ROPE TO TREE. The commands are interpreted by the parser and the appropriate response is displayed. The player then types in another command and the cycle continues, with the player exploring the environment and solving puzzles. Most IF is not in real time (i.e. the game will wait indefinitely until a command is entered) although there are exceptions. Some IF games do employ graphics, for example an image of the current location, but these are not necessary to play the game.
The game world is made up of a number of discrete locations ('You are standing at the end of a road before a small brick building') which may contain various objects ('There is tasty food here') that the player can manipulate. Movement from location to location is usually by typing in cardinal directions ('GO NORTH'). The new location is then described, any objects present are listed, as well as anything else relevant happening in the game world ('The dwarf throws a nasty little axe at you, misses, curses, and runs away').
Common IF Commands
Some of the more common and generally universal commands are:
Movement commands - GO NORTH (EAST, SOUTHWEST, DOWN etc. - usually abbreviated to N, E, SW, D etc.), IN, OUT, ENTER HOUSE, GO THROUGH GREEN DOOR etc.
LOOK (also REDESCRIBE or just R) - describes the current location.
INVENTORY (usually abbreviated to INV or I) - lists what the player is carrying.
GET or TAKE - pick up an object.
DROP - drop object.
EXAMINE (usually abbreviated to X) - get more information about something.
BRIEF, VERBOSE - the level of detail shown when a location is described.
SAVE, RESTORE - save current game to be resumed later.
QUIT - end current game. Action commands - PUSH, PULL, MOVE, OPEN, CLOSE, LOCK, UNLOCK, WEAR, REMOVE, SEARCH, BREAK, TIE, LIGHT etc. etc. etc.
This transcript is taken from the Inform version of Countdown to Doom by Peter Killworth, slightly modified and originally published in his 1984 book How to Write Adventure Games. Player entries are in caps.
To explore further you can use the sub-menu on the left...
What my favourite IF games are.
A guide to playing IF on modern PCs.
Applications available for writing IF.
A brief history of IF.
The various companies that produced IF during its heyday. MUDs (multi-user dungeons) are somewhat related. Think IF crossed with MMORPGs and you get the general idea.