Screenr is the easiest way I have found to create and share screencasts. You can sign up using an existing account from Facebook, Twitter, Google or Yahoo and then then just click the record button to get started.
You will be given a box which you use to display the part of the screen to show. Press record and start talking and pause at any time. Press done when you are finished and it will then process the screencast for you to share with others.
This is a great tool which could have many uses in the classroom. Teachers could easily create and share tutorials with students and students can even create their own tutorials to explain something they have created or learned.