Whoever designs your ebook cover will send it to you at 72 dpi, which is lo-res, the resolution needed for viewing it on a computer screen or ereader. But if you want to print promo materials with your book cover on them, you'll need a high-res version of it so that it won't be fuzzy.
I asked for one and the designers sent it to me with no problem.
If your designer uses an unusual font for the title on your front cover, be sure to ask the name of it. You may want to download it to have for promo materials. Even if you don't design these yourself, you'll want to tell your designer the name of the font.
Be sure to get a link to the page where you can download it. (Remember, some fonts are free and others are not.)
Ebooks have front covers only--no back cover. In a traditional bookstore, back covers are what customers read while they're looking for reasons to buy your book. Since you don't have that on an ebook, you have to make sure your promo copy has all that and more. That's why describing your book is so important.
Have some of your own tips or helpful advice you discovered when designing the cover to your ebook? Please share them with the rest of us by leaving them in a comment below.