The difference for making an Andriod and IOS game?

Hi I have been working with Unity for about a year now trying to make game for the PC, and I wanted to develop a game that can run on the IOS or Andriod. Is there a difference behind making a game for different platforms?

When using unity I see that I can build it for an Andriod or IOS. Is that all I have too do? If I, for example, make it first for IOS, do I just change the build settings to Andriod and it will work? Or is there platform specific code that I would have too change?