You only need to download an ebook if you want to read it in a mobile device or if you plan to use it when you are not connected to the internet. In most cases, you can read the ebook and make notes and print selections all from within the ebook platform interface, without having to download the book.
DRM systems control the use of digital content, such as downloadable songs, DVDs and CDs, games, and of course, ebooks.
The library's biggest ebook collection that uses DRM is EBSCO ebooks
You can download DRM-protected library ebooks onto your computer or your laptop, but you need to install Adobe® Digital Editions (ADE) software.
ADE does NOT support any Amazon Kindle e-readers
This means that you CAN'T download any DRM-protected book onto a Kindle, UNLESS you are able to use a non-ADE DRM intermediary (for example, EBL ebooks may be read on the Kindle Fire using the Bluefire app).
Video -- Get Adobe Digital Editions (ADE), Then Transfer a Book from ADE to Your Device
This video will show you how to install ADE software on your computer or laptop, download a DRM-protected ebook into ADE, and then transfer that ebook onto an e-reader (in this case, a Nook).
The video is from another university, so ignore the part in the middle about their catalog
Help for specific e-readers: YouTube offers many good videos on how to move ebooks onto different e-readers