Median Bearded Iris are the smaller classifications of iris. They come in a few different shapes and sizes as you can see in the diagram below. The best reasons to add them to the garden is the earlier bloom season and the shorter sizes. They tolerate wide varieties of conditions with the MDB's and SDB's being much more cold tolerant (not as heat tolerant) and ideal in rock garden type conditions.
The IB's bloom next and fill in the garden until the tall and border bearded iris bloom. Often, IB's and BB's are a combination of SDB's crossed with tall beardeds, giving them more tolerance of weather conditions.
We are big fans of these babies and highly recommend trying some in your garden. For more on the medians go the Median Iris Society website. We offer many different varieties at our own site, Stout Gardens at Dancingtree.