ISBN stands for International Standard Book Number, and it will be the main identifier for your book. Again, you don’t have to make a decision right now on which type of ISBN you want, but as soon as you’ve started the uploading process at CreateSpace it will be a one time decision that cannot be changed without beginning a new project. This is a great link for understanding the 4 current ISBN options: https://www.createspace.com/Products/Book/ISBNs.jsp.
The imprint will list the publisher’s name. The free ISBN option at CreateSpace will list CreateSpace as the publisher. All other options will list whoever you want as the publisher, either you or your publishing company. There are both $10 and $99 custom versions where you can purchase an ISBN through CreateSpace and be listed as the publishing imprint. There is a fourth option where you use an ISBN you already purchased at Bowker or another venue.
Different authors have different needs when it comes to ISBNs. Should you go for the free one, buy one from CreateSpace or provide your own from elsewhere? The choice is yours as this decision divides many authors. Books can be successful even with the free, CreateSpace assigned ISBN. The free ISBN actually gives the most Expanded Distribution channels. The other options give almost as many Expanded Distribution channels, just a bit less for the library and academic institution outlets.
However, it is my belief that if you plan to aggressively market your book to bookstores, it will be better to buy your own ISBN whether through CreateSpace, Bowker or another retailer.