While aimed at libraries with iPad loan programs, this guide has links to manuals and suggestions for apps for your iPad.
An ereader is a device whose primary purpose is to display ebooks and audio books in electronic format. Tablets are handheld computers that are capable of many things, including displaying ebooks and audiobooks.
From the website, "A mobile device is basically any handheld computer. It is designed to be extremely portable, often fitting in the palm of your hand or in your pocket. Some mobile devices are more powerful, and they allow you to do many of the same things you can do with a desktop or laptop computer. These include tablet computers, e-readers, and smartphones."