Paradox Interactive has been well known for making most of the games they publish highly moddable and Cities Skylines is no exception. Colossal Order, the developers of the game, have created a few tools to help modders up such as a map editor where you will be able to do normal map editor-y things such as making rivers, forests and mountains.
The game will also make it easier to create buildings and put them in the game. This will be partially done through what the developers call an asset importer where you can take objects (likely from in-game or built by yourself) and you will be able to manipulate them by playing with textures and dictating their functions and purposes when inside the game. Of course, just like any game with moddability, Cities Skylines will be utilizing Steam Workshop so that you can share content you create.
Cities Skylines is a city-building game being developed by Colossal Order, the guys behind the cult-hit public transportation simulator series Cities in Motion.