Most people today, do not know the difference between a digital book and an ebook. In fact, many people think they are one and the same. They could not be farther from the truth. They are entirely different species of the same animal.
Digital books, sometimes also called electronic books or PDF books, are scanned, digital facsimiles of standard printed, published books. You can think of them as enhanced copies of the actual hard or paperback books we have come to know and love.
When we are talking about the digital version of newer publications, they are pretty much identical to the original. When talking about old or even ancient publication, they are much better than any original you would find on the shelf of your local library. Since they are facsimiles of the original printing, this really increases quality and helps overcome the Challenges of many older books, such as yellowed pages, stains, see-through or onion skin type paper, colored paper, brown inks, etc. These are scanned pages, not 'copied' pages, and the quality of print really represent a better quality than the print of the original book.
The great majority of digital books come in PDF format, though Amazon offers a Kindle edition, Mobipocket offers the MOBI version, then there is the plain TEXT format, as well as many others.
Digital books are usually far less expensive than their hard or paperback counterparts, and there are hardly ever shipping costs, unless they are delivered on a CD (Compact Disk).
Ebooks are digital books too, but they were designed and written for the internet. Many, if not most ebooks were never officially "published" at all, unless you count posting something on the internet as publishing.
Most ebooks were meant to be written quickly, with little or no expenses except for possibly distribution. The majority is short, almost always less than 100 pages, usually under 50 pages. Some ebooks are literally slapped together in a matter of hours. Often they are little more than several short reports combined together.
Many ebooks are self-help books, or manuals of some kind, though there are some eBook works of history and fiction to be found if you look for them. More often than not, they have little or no literary value, their intentions being the distribution of facts, instructions and / or ideas.
Sometimes you may come across the eBook version of a hard or soft cover book. It will even be called the "ebook version" of whatever book. But if you look closer, you will find that this eBook version is almost always fewer fewer pages than the original. You may as well call the "ebook version" the "condensed version" of the book.
Although you will find many ebooks in PDF format, much like digital books, but many come as various generic interactive applications. But do not let the initial attraction of interaction fool you. Even if this sounds like high tech education at first, you will find that there is not much to the promise of interaction. It just disguises the fact that these books are short, technically no more than a few pages, with little to no real practical, and even less academic value.
Last but not least, ebooks are often free or cheap, although you will occasionally find some specialty instruction "programs" costing hundred of dollars.
Technically, ebooks are digital books, although practically there is a big difference. Personally, I usually prefer a digital book to its eBook cousin.