Diff between Invoke by Query Vs Invoke by Operation

Hi Guys,

       I am new to mobility.  I have done some sample application following the youtube, but I am confused as When we should use Invoke by Query and When we should use invoke by operation?

I tried searching for document to see the diff between both, but could find it.

Also do we have iphone/ipad simulator for Windows? where can we download it?

Appreciate your help,

thanks,

GN