There are several apps that will let you share a presentation (photos, PowerPoint presentations, etc.) and record your live annotations and voice notes. I want to share information about two of these apps below, but there are several others available in the app store.
First up is Explain Everything. This is a paid app ($2.99) but with all of the recording and import/export options it is well worth the money. When you create a new project with this app, you are given the option to import a PowerPoint, PDF, or photos from your iPad or from your online file storage program, like Dropbox. You can also start with a blank screen and add your own notes and annotations. Once your recording is completed, you can save the movie file to your iPad or share it via Youtube.
Another great app is Educreations. It is free and they are adding features all the time. Any presentations you record are saved to your iPad and can be shared via email or via a link to your video on their website. The Educreations website is also a community of other teachers who are creating and sharing their videos, so you may be able to find other presentations that work for your lessons.
See the images below for a comparison of the recording screens in Explain Everything and Educreations:
In Explain Everything, the primary controls and settings are listed to the left of the recording screen. The page selector, playback, recording and saving options are listed at the bottom.
In Educreations, the controls for recording and other controls are displayed above the recording screen. You can use the controls at the bottom of the screen to change the paper, duplicate and add pages.
BONUS App: Knowmia Teach is also Free, has a database of teacher-created content similar to Educreations and has a ton of great recording tools like Explain Everything. It also allows you to use the front-facing camera on your iPad to record your face while you are teaching to make the videos a little more personal. See more about Knowmia Teach, including some great screenshots on the Knowmia website or in iTunes.