When you are not actively using an application on the iPhone, iOS puts it in a suspended state until you go back to it. It essentially freezes the app (notable exceptions are native apps and things like Pandora.) When they say "real" multi-tasking, the other platforms allow the programs to actually run in the background. While this allows the phone to do more things at once, it swallows up a lot of system resources and naturally consumes more battery and memory.