I'm a BIG evangelist for screencasting ... on all devices ... IMHO it's an essential skill to go into every teachers and learners digital toolbox.
On the iPad we are spoilt for choice, many are free ... if you are looking for a suggestion on where to start then download Educreation (free) ... it is similar to having a single interactive whiteboard screen that you can put images on, draw, type and talk ... with it all being recorded for use later .. often the resulting video can be uploaded to e.g. YouTube.
Another of the Apps in my collection below that deserves extra attention is Coach's Eye ($4.99) this App is designed for P.E./Sports staff to video sports actions and then play video back immediately with the option of writing on the screen and annotating the action ... a MUST for your P.E. colleagues.
But there is now a new guy (app) on the block ... Display Recorder ($1.99) ... this does something that no App has done before (unless you had a jail broken iPad/iPhone) ... and that is to record anything that is happening on your iPad ... with no restriction that it needs to stay on one screen.
One of my more requested teacher prof.dev. workshops is all about Screencasting, below is the interactive visuals I use in the workshop.
If you'd like me to do it in your school then please drop me an email.