Compiler definitions run platform specific code. Using them you can make small differences between various platforms.
void Update(){
#if UNITY_IPHONE
//code here is only called when running on iPhone
#endif
#if UNITY_STANDALONE_WIN && !UNITY_EDITOR
//code here is only ran in a unity game running on windows outside of the editor
#endif
//other code that will be ran regardless of platform
}
A complete list of Unity compiler definitions can be found here