Screencasts are an efficient way of showing someone else how you are performing tasks in your operating system. They are mainly used for educational purposes such as recording step by step instructions on how to use a certain program by capturing a certain region of your screen and commenting the actions using a microphone.
The service Screencast-O-Matic offers a free and reliable way of creating screencasts using just your browser and Java. It is not necessary to register an account to create a screencast. This is only necessary if you want to publish the screencast later on the website of the developer.