Playlists allow a content owner to group content together for embedding on an external web page. Every time you add a new or remove an item to a Playlist, your embedded player automatically updates to reflect this change.
Channels allow a content owner to group media together in MediaSpace. Additionally, the content owner can restrict access to that channel to a defined list of users (based on ONID usernames).
Categories are created by Kaltura systems administrators, and media added to a category are automatically publicly available. Note: if you publish your media to a category, adding that same media to a Channel will not restrict who has access to it.