ArcGIS Online is an online, collaborative web GIS that allows you to use, create, and share maps, scenes, apps, layers, analytics, and data. You get access to Living Atlas of the World, apps, and Esri’s secure cloud, where you can add items and publish web layers. Because ArcGIS Online is an integral part of the ArcGIS system, you can use it to extend the capabilities of ArcGIS Desktop, ArcGIS Enterprise, ArcGIS Web APIs, and ArcGIS Runtime SDKs.
ArcGIS Online is a cloud-based software that allows you to organize and create GIS projects, which can come in the form of story maps, GIS files created with desktop applications, web maps (more on those below), and more. ArcGIS Online offers two tiers of accounts: Public and Organizational. Public accounts are free and they grant you access to most of the features in ArcGIS Online, including the ability to create web maps, story maps, scenes, apps, and public groups. An Organizational account grants you extra privileges, including access to Living Atlas Layers, built-in analyses for Web Maps, uploading hosted layers from GIS files, and more.
An ArcGIS web map is an interactive display of geographic information that you can use to tell stories and answer questions . . . [Web] Maps contain a basemap, a set of data layers (many of which include interactive pop-up windows with information about the data), an extent, and navigation tools to pan and zoom. In general, the basemap and layers are hosted and shared through ArcGIS Online. However, maps can also contain layers added directly to the map and layers and basemaps referenced externally. Many maps also contain scaled symbols and other smart styling that reveal data and patterns as you interact with it. For example, the map below shows the relative magnitude of earthquakes that happened over the previous 60 days.
The web map referenced above:
Web maps are cloud or server-hosted maps that are essential for story maps and other ArcGIS Online applications. Web maps are the foundational element to any story map. Depending on their complexity, web maps can be created purely on ArcGIS Online's Map Viewer, or in conjunction with ArcGIS Online and ArcGIS desktop clients. The Map Viewer allows you to create and edit web maps, and it incorporates a variety of built-in tools, like public-access map layers, statistical analyses (only available for organization accounts), and a host of visualization options. One of the most useful and commonly-utilized features of web maps are pop-ups, which allow users to click on symbols or areas on your map and read more information about your specific point, shape, or line. As an example, the symbols on the embedded map above are pop-up enabled.
When you create a new story map and save it for the first time, two files are created: your story map and your web map. The story map is a web mapping application, and it contains all of your narrative work -- your text, images, videos, and story sequence -- and the web map contains merely the points you placed in building your story map. When you view a story map, any interactive maps you see are web maps referenced from somewhere else on ArcGIS Online. Web maps are not stored inside story maps in order to improve load times for story maps that contain many web maps.
Web maps also give you some flexibility in your story map, because you can access the underlying web map via your "contents" page and fiddle with the points you've created. You can also use the array of features that the map viewer offers, which are listed above.