Unity’s Terrain system allows you to add vast landscapes to your games. At runtime, terrain rendering is highly optimized for rendering efficiency while in the editor, a selection of tools is available to make terrains easy and quick to create. This section explains the various options available for terrains and how to make use of them.
See also the Knowledge Base Terrain section.
Is something described here not working as you expect it to? It might be a Known Issue. Please check with the Issue Tracker at issuetracker.unity3d.com.