They're visual aids, not textual aids. Treat each screen like a billboard. One large and bold message. Relevant, colorful, informative images are best. Use as few words as possible.
Movement is good.
In PowerPoint, you can build screens item by item. Both with PowerPoint and on the Web, you can use animations and videos and still images. You can use screen shots. You can use concept maps and other visualizations such as charts and graphs. You can make vivid comparisons.