DRM means Digital Rights Management and is both a mechanism and an idea. It generally intends to enforce a restriction on a user in what they can do with a particular file.
DRM as applied to eBooks is generally a code that must be present in order for the reader to be able to open an eBook. The code may be locked to a particular device, or may be locked to a range of multiple devices. In other cases it is locked to a user. There is no standard implementation for DRM.
DRM is supposed to protect the author and publisher of eBooks (or eMusic and other electronic media) from pirating and other illegal activities. Some opponents call it Digital Restriction Management and claims it goes far beyond illegal activity and restricts items that are both legal and reasonable use.
Some eBook formats do not support DRM and even if a format supports it there may be eBooks in that format that are not controlled by DRM.
A given reading device may have multiple DRM schemes for the various formats it supports. Some formats, most notably MOBI have a restriction on dedicated reading devices that they will not permit their software to be used on a dedicated reading device that supports other formats using DRM, although more general devices with eBook reading applications that are loadable can often have MOBI and other reading programs.
Generally DRM uses a server to administer the DRM scheme. When a user buys an eBook that is DRM protected the server assigns the decryption key which could be keyed off the reading device itself or a user id known to the server.
 PDF DRM
Adobe developed the PDF format and is the dominant player using DRM to control access to these documents. There is also password protection that is used by some systems for PDF content control.
 ePub DRM
While ePub is an industry standard that is generally well defined they did not define a method for even identifying DRM content. There is no standard implementation for this format. In practice there are three major systems currently in use.
Two of the systems are managed by Adobe. The Adobe DRM server is used by Adobe Digital Editions which used by most dedicated eBook Readers, and Barnes and Noble used for the nook. The third system is done by Apple via their iTunes application. For more information see Apple's FairPlay DRM.
The real problem for the user is they have to be careful when they have a device that can support more than one reading program. The systems available do not recognize the existence of another form of DRM and may behave strangely if they encounter it. Often there is no error message but only a display of a blank screen.
To make matters even more complicated for the user the DRM scheme in ePub is normally applied to a subset of the files inside the container. This can make an eBook seem to partially work in a reader as some things may appear normal while other things do not.
One of the most confusing partial use is when internal embedded fonts specified in the eBook are encrypted while other eBook elements are not. This can happen when copyrighted fonts are being used. Another technique that is less obtrusive to the user is to use obfuscated fonts which use and XOR (exclusive or) technique to obscure the fonts in an embedded font set so that it cannot be extracted and used by itself. This can sometimes be used to meet the copyright requirements imposed by the font designer.
 Amazon formats
The Amazon Kindle line of products has many different proprietary formats, all of which support a DRM scheme. The DRM is applied directly to the container format, AZW, which is based on Palm PDB format. This is the basis for MOBI which Amazon purchased and uses this same basic scheme for all of their various formats. Without DRM differences the MOBI format and the AZW format are basically the same. Internally the format of the raw data can be very different. For example AZW4 contains PDF content wrapped in the Amazon container while KF8 contains ePub content. TPZ, also known as AZW1 is another Amazon format containing OCR'd files.
 Other formats
There are many other DRM schemes for eBooks and other formats. Microsoft has DRM for their eBooks (LIT) and music (WMA) and even video files. Apple also has or had DRM for their eBooks, music (AAC), and video files.
Most other DRM schemes are specific to formats only supported by the owner of the format. For example MOBI had its DRM scheme and Kindle has its DRM scheme (which is different from MOBI although similar). Kobo used Adobe DRM for its regular ePub books but also uses an internal form for those that are stored in its kepub database. Several less popular formats have their own schemes.
 Library Use
DRM is often used by libraries that loan eBooks and in this case the DRM offers a time limited license to the person checking out the book. The library monitors the number of licenses it purchased and controls checkout just like a physical book except that there is no need to return the book since it will expire.
Current eBook formats that support time limited checkout include PDF, ePUB and AZW (used to be MOBI but was switched by Amazon for the US at least). Audio formats that support time limited checkout include secure WMA. The support for Libraries is done primarily by Overdrive.
See EBook Lending Libraries for a list of Libraries and eBook reading devices that support library checkout.
 Font DRM
Many Fonts are separately copyrighted. Those that are embedded in eBooks can also have DRM applied. Sometimes the eBook won't use DRM but the fonts may still be protected. Instead of traditional DRM some embedded fonts will be obfuscated to prevent their use outside the eBook.
 Impact on TTS
Visually impaired readers often use a TTS (Text To Speech) engine to read the text on the page for the reader. This ability is also used by readers who are attempting to read a book that is not in their native tongue to help learn the language. However DRM can have an impact on the use of TTS since it may block external applications from being able to use the data. For more information see Daisy publication. Note that the disabilities act in the USA provides provision of legal access (including breaking DRM) to permit reading books which is in favor of TTS but if the eBook is also sold as an audio book then DRM may be used to disable the TTS feature. Note that this applies only to a disabled user needing this feature and is not carte blanche permission to break DRM.
 Additional restrictions
DRM can also prevent being able to print even a portion of a book and can also limit the number of times the book can be read.
 Removing DRM
In some countries it is not legal to remove DRM from an eBook. This related to copyright in the sense that it is illegal to copy or distribute copyrighted material without permission except in certain limited cases, such as quoting a section in another work. In the USA the DMCA act prohibits DRM removal. However, proponents of the doctrine of "fair use" feel that removing DRM is a reasonable way to allow fair use. Typically a program that removes DRM requires that the user input the correct ID that was used to apply DRM in the first place, so the program cannot be used to universally remove DRM from any eBook in that format. The distribution of a program like this and its use may be illegal in the country you live in. Typical target DRM encrypted formats include LIT, MOBI, Adobe's version of EPUB, some Adobe PDF and eReader and Barnes and Noble's version of eReader and ePUB. There is no known method of removing DRM from other common formats such as LRX.
 See Also
- http://en.wikipedia.org/wiki/DMCA Digital Millennium Copyright Act Wikipedia article
- http://en.wikipedia.org/wiki/Digital_Rights_Management Wikipedia DRM article
- http://www.copyright.gov/title17/92chap12.html - the chapter on DMCA as related to DRM from the US government.