Most features of the Android devices are exposed through the Input and Handheld classes. For cross-platform projects, UNITY_ANDROID is defined for conditionally compiling Android-specific C# code.
For further information, see the Input, Mobile Keyboard and Device Property pages.
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.