Writing down the beliefs your startup is based on is crucial... it keeps you focused on risks instead of features, makes communication w/ team & investors easier, and stops you from ignoring the evidence. Yet most people don't do it (and almost no one consistently updates them).
I thought Steve Blank's customer development worksheets were a good start, but it's hard to update quickly and doesn't provide an overview. The canvas you see on the app is a modification of the Business Model Generation canvas, which I re-vamped to better suit startups.
Hope it's handy.