What's the difference between a factory unlocked and a network unlocked cell phone?
Nothing, AFAIK. Factory unlocked won't have the network software installed, while network unlocked probably will. For instance, I unlocked my Samsung Galaxy from AT&T so I could use it with T-Mobile. My AT&T software in still installed, but not used now. Also, I kept my AT&T SIM card, so I can switch back if I want.