Workspaces are the main container for everything your team does. All Projects, Actions, Messages, Groups, and Users fall under Workspaces.
Actions in Hive represent pieces of work for you and your team to do (similar to Tasks or To-dos like in other apps, but much more powerful and team-oriented!). Actions always belong to a single Workspace and can belong to either one or no Projects. Lastly, part of what makes Actions in Hive so powerful is that they can be infinitely nested within each other. Actions that are nested are called sub Actions and are denoted by their "parent" field having an ID.