I am looking into using OneNote. Will it only be on a single computer PC, Laptop or Pad?
Can I use it on something like DropBox?

OneNote works well on OneDrive, MS's cloud. There are a few issues, but all in all, notebooks are available to all devices, and they are relatively easily managed. Since I keep everything in OneNote, it's often handy to use the cloud function. Had to look up the ingredients for a martini I'd had at a restaurant a few months ago; searched on keyword "martini," and found the document I'd within seconds. Of course, I use ON for more than keeping drink recipes.