Conformation is the way that a donkey is built or put together. If you plan to breed selecting breeding stock with great conformation should be a top priority. The ideal is to intentionally breed a pair to produce an animal that exhibits the best traits and characteristics of each parent. Responsible breeding programs like this will continually improve the breed.
If you are looking to add a donkey as a pet certain conformation shortcomings are tolerable and will not affect the health or function for your donkey. For example, donkeys will often be a little cow-hocked. This does not hinder the usability of the donkey.
Cindy Benson, owner of Benson Ranch, provides great advice on what to look for when selecting a donkey in her article, 'A Field Guide to Evaluating Miniature Donkey Conformation'.
The Donkey and Mule Society of New Zealand, Inc. has also written a comprehensive guide, 'Standards of Conformation & Type for Donkeys.'