On the free eduClipper iPad app you can create instructional videos on a whiteboard in the Khan Academy style. You can also use the app to create a video in which you annotate an image or document while talking about it. After creating your video you can save it to an eduClipper board that you have shared with your students through the eduClipper classroom setting. Your students can view the videos on their iPads or in the web browsers on their laptops.
Whiteboard and Annotations from AdamBellow on Vimeo.
If you don't have an iPad, PixiClip is a good option for creating simple instructional videos. PixiClip provides a whiteboard space on which you can draw, upload images to mark-up, and type. While adding elements to your PixiClip whiteboard you can talk and or record a video of yourself talking. In fact, you can't use the whiteboard without at least recording your voice at the same time. Recordings can be shared via social media, embedded into blog posts, or you could grab the link and include it in an eduClipper board that you have shared with your students.
Disclosure: I am an advisor to eduClipper with a very small equity stake in the company.