Scored Levels are "puzzles" for players to try their hand at. Tracks, Signals, and Contracts are set in stone, and you must make sure trains keep to their timetable as best you can. You can read more Here.
Freeplay Levels are levels where the player has the power to build as they see fit, and has complete control over the contracts they manage. Instead of getting points, players get money, and can spend that money on system upgrades. You can learn more about them Here.
The Level Editor allows players to design their own scored levels or freeplay maps with a plethora of tools to refine your creations.