A list app store rows of data much like a table in a database or a worksheet in Excel. A row of data is comprised of one or more columns that can host a variety of types of data. The app allows users to create views, apply workflows, manage access, customize edit forms and subscribe to receive alerts.
A library app stores documents and other files much like the file system in Windows. Columns, metadata, can be added to the library to tag each file as they are uploaded. As with the list app, users are able to create views, apply workflows, manage access, customize edit form and subscribe to alerts.
A site is a web site that contains apps and pages. Sites can be created for an organizational unit such as departments or divisions, projects, processes, complex documents, events, meetings, etc. Sites are stored within site collections. Users can manage access to them and activate or deactivate features. Sites can be deleted or moved and archived.