A presentation and a collection are both a custom group of content. You can add ViewMedica videos, slides, and your own videos to both. The difference between the two is how you interact with them.
With a presentation, you click play and it plays all the items in the group from beginning to end.
A collection creates a custom menu with all the items in the group. This allows you to customize the navigation and choose which content item to watch.
Presentations don't loop. To loop custom grouped content, check out VMcast.