A resource is a single 'Lesson'. It can be made up of many elements (videos, quizzes, etc) and can be formatted as a single page (like this one) or as a slide-show.
A theme is a sub-category of a workspace where you can organise your resources into sections.
A workspace is a collection of resources. Every resource must belong to a workspace. User access to resources is controlled through workspaces.
A pathway is a 'playlist' or 'course' made up of many resources. Pathways can be 'linear', forcing users to complete the resources in a particular order.
A face-to-face or online training session is called an 'event'. Events have times and places, and people are invited and can RSVP to book their spot.
A campaign is an automated workflow that allows admins to set up a sequence of actions triggered either on a specific date/time OR a specific number of days 'into' the campaign. Users are added and removed from a campaign 'audience' via filters. When a user first matches the filter they are added to (start) the campaign, and when they no longer match they are removed.