In Interactive Fiction, the player interacts with and influences the outcome of a story by entering commands in a text interface, the "parser".
Example conversation:
You stand in front of a house. You can see the front door
which is closed.
> open door
It appears to be closed.
> look under the door mat
You didn't think it would be that easy, did you ?
Most works of Interactive Fiction share a common dictionary, which is usually generated by the authoring system whey were created with, but a lot of them also define a set of story-specific words that invite the player to experiment. Common commands are e.g.
open
look
north, south, west, east, up, down
etc. (for player movement)
inventory
save, restore, undo
(for saving/restoring your progress)