A Scenario or Landscape in OpenTTD is the area on which the game is played. Every scenario is different, and basically features land and water. New scenarios can be randomly generated, or already existing ones, human designed, can be downloaded and played (a few are bundled with the game). Alternatively, you can choose a setting and design your very own scenarios with the Scenario editor. There are also a few tools where scenarios can be generated from Heightmaps. These are often used to represent real life geography.
Scenario editing originated in Transport Tycoon World Editor, where scenarios were referred to as landscapes. Scenarios have their own file extension, .SCN, and are loaded from and saved to the /scenario directory.
To load a scenario from the Command line, use: "-g scenario/<name>"
Scenario resources
For a list of scenarios bundled with OpenTTD, see List of scenarios in OpenTTD.
For a list of downloadable player-made scenarios, see Scenarios.
For information about and a list of "Scenarios Team" projects, see Scenarios Team