Presentations has some of the basic functionality you would expect from a slide show app, with a rich text editor, images options, and a few slide layout and themes to choose from. But it lacks many of the features that businesses and those that rely on PowerPoint for important presentations will probably want.
That said, the collaborative editing features of Presentations are really the service's strong point. Just like with other Google Docs applications, you can pick a list of collaborators who will all be able to work on the same presentation. When you are done, you can either share it via a public URL or present it to a group of people that you invite.
Compared to PowerPoint, Presentations still has a long way to go. Nevertheless, its refreshingly simple interface and feature set might be enough for users who just want to get something done quickly.