DRM - Digital Rights Management:This is technology that purposely limits the use and duplication of copyrighted works. For example, DRM may limit the number of times an ebook may be read; what type and number of devices the file may be used on; and whether or not a full or partial copy can be made of the file. DRM-free titles are great - no restrictions!
Open Standard file formats can be used by any company without paying royalties or licensing fees.
Proprietary file formats can only be used by the creating company (example: Kindle), or, if that company permits, by third parties that have purchased a usage license. File formats include:
eBook readers: devices designed primarily to read digital books.
Mobile devices: such as smartphones and tablets. eBook reading apps are usually available for download to optimize reading on the device.
Computers: including PC's and Macs, a variety of desktops and laptops, tablets, etc. Apps are also available to download to read ebooks on computers.
Note: Different devices will accommodate varying types of ebook formats and ebook reader apps ideally or not-so-ideally. For an ebook reader device to display an ebook, both the file format and DRM must be compatible with the ebook reader. So, for example, an iPad can only (theoretically!) view a book from Amazon protected with Amazon DRM if the Amazon Kindle app is installed on the iPad.