Trying to Understand iPhone Software Hacks

I am trying to understand iPhone hacks, mostly so I know what not to buy, as I am buying and selling a lot of iPhones these days. I am getting pretty good with iPhone hardware, but some of the software hacks still boggle me.

I understand you can unlock and iPhone without jailbreaking it but from what I gather this process isn't seamless. For example you will need to re-unlock each time your turn the phone off?

If you jailbreak a phone, you cannot return to the factory Apple software?