Rover's Day Out is a strange piece of interactive fiction, that you can play using an interpreter program like Gargoyle.
For newcomers, there is nothing graphical about interactive fiction - the whole game is text based. You can interact with your world by typing commands, like look, examine and pick up. There is no help with the commands, but there is a guide to command principles on the interactive fiction website. Rover's Day Out is fairly typical in terms of what commands you use, and you'll soon pick up how to play.
What makes Rover's Day Out so interesting is the story being told. At the start of the adventure, it's not clear what's wrong, but you get the feeling everything is not what it seems. The story is very much science fiction, and it's well done and creates a convincing world in which to tell its unique story.
Due to the strangeness of the story, Rover's Day Out may not be particularly accessible, and as with many pieces of interactive fiction, getting stuck is very frustrating. If you're don't know what commands will work, there is nothing to help you in game. Rover's Day Out does come with a walkthrough, so you can get help when you need it, but it would be better if it were more accessible to begin with.
Rover's Day Out is an excellent piece of interactive fiction, and makes a great change of pace from loud 3D games. If you enjoy puzzles and reading, you'll love it.