Educreations: This app in its free version is great for basic demonstration of processes and ideas. There is a recoding option where you can save a presentation and replay it at any time. The free version will not allow you to share the file. This is an option in the upgrade however. There are options to choose typewritten text, upload pictures, take pictures, customize the the background. You are limited in color options to the basic primary colors (black, blue, red, yellow, and green).
Showme: is another free app that allows you to create presentations using similar features as Educreations. There are a few differences. Showme creates a community and adds your presentation to it. I enjoy the organization aspect of Showme. You have the option to create courses and follow other members.
Explain Everything: has yet even more user options for your demonstrations. I enjoyed being able to zoom in and out as you are viewing your pdf or in app doc. The laser dot option is a great feature to use while a teacher or student is showing their process. Explain Everything give you insert options to add a photo, video or file, existing sound file, math equation, or web page/browser page. What is cools is that you can resize and change the orientation of any of these insertions. The upload option allows for quick access to various locations such as: photos, iMessage, dropbox, Youtube, Google Drive, OneDrive, Evernote, Vimeo, and Mail. You can export as PDF, video, Image, and Project.