ArcGIS Online is ESRI's online tool for creating web maps in the cloud. ArcGIS Online provides a way for users to create web maps, mix and match map services and other data sources, modify popups, and share the resulting web maps, either with the general public or within their organizations. This is a screenshot of the ArcGIS Online page where you can add data from different sources into custom maps:

Besides referencing data through web maps, ArcGIS Online subscriptions allow you to upload and store data. Tiled map services can be created with tiled map packages generated through ArcGIS Server. Feature services can be created to store, update, and retrieve feature data. You can also use other ArcGIS Server extensions, such as geocoding and routing through ArcGIS Online. This means that you don't have to use your own hardware to host your data. You can maintain your feature data and tiled maps in the ArcGIS Online cloud.

Now, I'm sure you're wondering how ESRI can afford to offer...