Any notes containing other notes, i.e. in outline terms having child notes, is considered a 'container.' At the most trivial level this allows notes to be nested inside other notes. For instance, a 'Section' container might contain a series of 'Chapter' notes.
Containers are most useful for structured projects that will mostly be viewed in outline or chart view. In map view, only sibling notes within a a single container are visible on the map. Containers on the current map draw a 'viewport' showing a part of its child map.
Agents are a special class of container, as their children consist of an alias to every note matching the query stored in their $AgentQuery attribute.
Adornments (map view only) are not strictly containers but act like containers in that they have OnAdd and OnRemove actions and support the inside() action query.
See also—notes linking to here: