Activity types are different content types, which you drag and drop from the to a page to add course content. The Activity types could be videos, documents, links, interactions, quizzes etc.
Here's a list of all Activity types with links to more detailed descriptions:
Text, checklists and callout.
Video (hosted or embedded from Youtube or Vimeo).
Images: Single images, Galleries and Carousels.
Interactions like Hotspots, Timeline, Tabs and Accordions.
Documents (pdf, doc, etc).
Hyperlinks to web pages.
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