What kind of condition is diaper rash?

It's a rash: Diaper rash is any rash, usually from irritation from skin contact with urine and feces, that occurs in the diaper area.

Zinc Oxide: After cleaning babies bottom allow to air dry completely before applying a barrier cream such as desitin (zinc oxide diaper cream) to keep the moisture away from the skin. I recommend this with each diaper change while rash persists. If it is particularly irritated you could also add a small amount of over-the-counter Hydrocortisone along with the barrier cream after diaper changes.