Diaper rash can be caused by a burning of the skin from the ammonia in the urine or acidity of the stool. The majority of diaper rashes are just in the contact-surfaces where the stool or urine has had contact with the skin. Barrier creams work great to prevent the stool or urine from having direct contact with the skin. Use the white diaper creams with zinc and apply it during every diaper change. Apply generously, like frosting.
The second most common rash is from yeast or Candida. Little babies may also have thrush (white on the gums, cheeks, and tongue). The Candida rash is typically in the creases of the thighs, around the anus, and for girls, also in the vaginal creases. I recommend that you alternate two different anti-fungal creams, one each diaper change. This will help to avoid the resistance that some yeast has developed to anti-fungals.
The final form of diaper rash is very rare. This is a peri-anal strep infection, and occurs in less than 1% of diaper rashes. Just like you can get strep throat, babies on rare occasions can get a strep infection around their anus. This rash tends to be beefy-red and will require a visit to your doctor for antibiotics.