Attached Link: GQ Gallery

This may be a stupid question, but is there a way to render in stages? I've read posts where people say it took 32 hours or 48 hours or whatever to render a pic and I'm wondering do they mean that they actually let their system render for that long or is there a way to let a pic render for a couple of hours, pause it/reboot/work on another pic then resume render? Anybody know what I'm talking about?