What is it?
Jason wrote on Signal vs. Noise about what Backpack actually is, and to be honest, it’s difficult to explain because it can be anything you want. It adapts to how you see it fitting into your life, and it can bend and reshape itself to fit the application you use it for. Personal notes, photo libraries, group organization, business information, scrapbooks. Multiple pages with multiple levels of functionality within each page equate to an application with customizability beyond that of all others. Think of it as a wiki, but overflowing with real-life usefulness and smooth interfacing.
I could discuss more about the functionality and interface, but it’s better if you just go check it out and roam around for yourself. Backpack is so flexible that describing exactly what it does is impossible, and that’s not a bad thing. Simple software takes the constricting details out and only leaves in the functionality you need, and Backpack’s simplicity shines right through its zippers.