All of our videos can be played on all devices that can browse the Internet (Apple, Android, Kindle Fire etc.).
You can watch the course videos online (i.e. like you can with YouTube) as often as you like and there is no time limit... as long as ArtTutor exists you have access to watch the videos online!
You can download the video files to your computer (PC or MAC) and to some Android devices. However, most mobile devices are not made for downloading files from the Internet and don't make it very easy.
The only way you can download a video to an iPad or iPhone is to first download it to a computer and then transfer it to your iPad or iPhone.You watch these videos via the iTunes app that comes pre-installed on every iPhone and iPad.
It's much better to do this anyway because iPads and iPhones have very limited storage capacity. Keep everything on your computer or an external storage device... and then transfer to your mobile device when you need it. Remove it afterwards so that you don't clog up the storage space.
Update: For those that more technically minded, there is an app available for iPads and iPhones that allows you to directly download files from the Internet. It's called GoodReader and is available from the App Store for a small fee. Be aware though that downloading large videos files will quickly use up the memory on your device.