eBooks are a relatively new technology, so publishers and authors are trying different formats to provide them for different purposes.
Part of the differences are based on DRM or Digital Rights (or Restrictions) Management to control the files so only authorized users can read them. This is to protect the rights of the copyright owners.
Wikipedia has a comparison of e-book formats with more detail.
Please note: ebook files may download with the ending ".acsm" (Adobe Content Server Message) but are actually in .pdf or .epub format -- they will load into your ebook reader like one of those.
pdf (Portable Document Format) is one of the best-known and most-used formats. File names end in .pdf for this format.
It comes in two "flavors" (so to speak): protected and unprotected. Unprotected .pdf files may be read by anyone with pdf reader software such as Adobe Reader and many other programs, and can be altered by anyone with .pdf editor software.
Protected .pdf files come in two subflavors: protected files that can be read by anyone with pdf reader software, and DRM-protected files that can only be read with specific software such as Adobe Digital Editions. Regular Adobe Reader will NOT read protected .pdf ebooks, so if your ebooks won't show up there, you will need Adobe Digital Editions.
While .pdf files keep the original layout of eBooks, they may not be quite as easy to read on small screens such as tablets, ereaders and smartphones. If you have a choice, .epub versions of ebooks may be easier to read.
Adobe Digital Editions can read both .pdf and many DRM-protected .epub files for eBooks. Some of both types are provided through the Library catalog.
Many older eBooks are still in .pdf formats, such as many of the Project Gutenberg eBooks.
The Library catalog also links to a number of .pdf eBooks and documents downloadable from websites, usually protected so they cannot be altered, but can be read.
epub (Electronic PUBlishing) format is a popular format for most eBook vendors. File names end in .epub for this format.
(Some vendors, however, alter .epub so their eBooks only work using their own ereader software.)
.epub files have the advantage of being able to reformat themselves to fit various sizes of screen, from desktop computers down to smartphones.
Boreham Library and Fort Smith Public Library eBooks are often available in a DRM-protected version of the .epub format, although the files show as Adobe Digital Edition ending in .acsm such as URLLink.acsm on your computer.
eBooks purchased from vendors other than Amazon are often in .epub format. This includes other bookstores, Google Books, public libraries, and other sources.
.epub ebooks are readable in a number of ereader softwares on non-Windows/Mac devices, including (just for examples) Bluefire for Android devices and Bluefire for iOS devices such as the iPad and iPhones. Many vendors offer their own software as well.
Amazon decided to use their own custom format for their eBooks, and their Kindle devices and ereader software are the only way to read them. This also allows them to restrict the use of other vendor's eBooks on Kindles. File names end in .azw for this format.
For example, Boreham Library eBooks are presently not readable on Kindle devices using the Kindle software. This is a decision controlled by Amazon and not the Library. If the Kindle has a browser, you may be able to read online, however, or Kindle Fire tablets may be able to load another reader.
However, Fort Smith Public Library uses eBooks through Overdrive, and some of those can be downloaded from Amazon as library loans, in .azw format, thanks to a recent agreement between Amazon and Overdrive. This applies only to those ebooks already licensed by Amazon for Kindle. An Amazon account is required.
The Kindle software is available directly from Amazon for PC, Mac, Android and iOS devices, among others. You will also need to set up an Amazon account, even if you only download ebooks for free from a library.
Many other formats are available, but the ones listed above are the ones you are most likely to encounter.
Some vendors offer a variety of formats, for use on different devices and software. You may select the format you prefer for the ereader software/device you have.
The Boreham Library recommends Adobe Digital Editions because the current standard is to use an Adoble I.D. to enable readers, and ADE is available for both iOS and Windows systems.
Bluefire is recommended because it uses the Adobe I.D. standard, and is functional for iOS and Android.
It is not practical to try to test and teach every possible device and reader app on every operating system, so these apps are a compromise. No reimbursement has been offered or provided.