Irritant dermatitis: occurs on convex skin surfaces that are in direct contact with the diaper, skin folds are classically spared. Treat by eliminating direct skin contact with urine and feces and applying topical barrier ointment ie zinc oxide (Desitin), petrolatum (Vaseline). In severe cases low potency topical glucocorticoids ie 1% hydrocortisone, may be used for 3-5 days.
Candidal dermatitis: if irritant dermatitis is left untreated, it may become secondarily infected with Candida. Candidal infections classically present with beefy red plaques and satellite papules; and in contrast to irritant dermatitis, commonly involve the skin folds. Treat by applying an antifungal beneath a barrier ointment.
Source
Horii, K. MD. "Overview of diaper dermatitis in infants and children." Up to Date. Oct 2008.