ArcGIS Online (AGOL) is one of Esri's web-mapping platforms along with the ArcGIS API for JavaScript, and Portal for ArcGIS. It is a complete platform, which includes cloud-based data storage, management, analysis, and sharing capabilities. AGOL has a free and a paid component, and storage and analysis beyond the free tier are managed using a credit system of Pay-As-You-Go tokens. Some contracts with Esri include access to ArcGIS Online and a set number of tokens to use, so check with the ArcGIS administrator within your company or organization to find out what you can do with an organization's AGOL site.
Within AGOL, there are a number of existing data resources published by other users, which can be incorporated into new web maps. With the free tier and subscription accounts, datasets and maps can be made available to the public so that other users can...