In Find My iPhone whats the difference between being offline and not being connected to the service?

A few days ago my iPhone 6 became lost/stolen, when I went onto Find My iPhone on my computer. My phones comes up as "Offline" but I'm assuming that who ever has it has just turned it off right? because I had my mobile data access turned on for the app so if it was on my phone would come up as online right? and how could someone restore the phone and erase everything off it? How do I tell the difference between my phone being "offline" or just not even being connected? please help