Is Atticus downloadable or web-based?

It’s a little bit of both actually. You can use Atticus in a web browser or install it as an app on your computer (or tablet). It also has some offline functionality, meaning you can write while offline and it will sync to the server once you are back online. You do need to be online to import and export files, however, and you'll have to make sure you're logged into Atticus before you lose your internet connection.