An image is fixed once you make it. An app is the opposite: a small program that shows information which keeps changing, without anyone touching it.
A dashboard that refreshes through the day, a calendar filling with the day’s meetings, a weather panel, a news feed, a clock. You give it a few details and it stays current.
Not just a web page
You could point a screen at a URL and get something similar. Two things make an app different.
An app is built to be shown rather than used. A web page is made for a person at a laptop: small text, menus, a layout that assumes someone is close enough to click. An app is laid out to fill a display and be read across a room, with nothing to interact with.
An app is also packaged for configuration. It declares what it needs, so the dashboard gives you a form: which dashboard, which calendar, which city. A web page gives you a URL and leaves the rest to you.
See Web pages for when a URL is the better answer, which is often, particularly for something you already built.
An app plays like content
To a screen there is nothing special about an app. It sits in a playlist beside your images and videos, takes its turn, holds the screen for its slot, and hands over to the next item. It follows the same schedule rules and plays on the screens you choose.
You manage it under Apps, not in Content. Everything you know about playlists and scheduling still applies.
What it costs
Nothing beyond your subscription. Apps are included on every plan, however many you use.
Categories in the store
The store groups apps into Utilities, Productivity, News, Dashboards, Just For Fun, Messaging & Content, HR, and Finance, with an Other bucket for anything uncategorized.
Several of the news apps are the same underlying reader pointed at a different publication, so if the feed you want is not listed, the RSS Reader takes any feed URL.