Dandruff consists of small, white scales that usally appear on the scalp and in the hair. The medical term for dandruff is pityriasis. Long neglected and excessive dandruff can lead to baldness. The nature of dandruff is not clearly defined by medical authorities although it is generally believed to be of infectious origin. Some authorities hold that it is due to a specific microbe.(shown below)

There are several different types of dandruff. The most common varieties are dry dandruff and greasy dandruff.

1. Dry dandruff or Pityriasis simplex
This is the most frequent, “flaky” dandruff state and usually lasts for long periods of time. If it is not treated, dry dandruff may reappear and become greasy dandruff.
Sometimes it spontaneously disappears. Its main characteristics are:
Presence of fine, dry, white or greyish flakes that completely cover the scalp.
The flakes are distributed around the neck and shoulders and have an unattractive appearance.
Dry scalp and dull hair.
There are no signs of inflammation and there is little or no pruritus.

2. Greasy dandruff or Steatoid Pityriasis
This flaky state is associated to the existence of sebhorrea, which may result from Pityriasis simplex or it may appear spontaneously. Its main characteristics are:
Presence of large, thick, yellowish or greyish, oily flakes, usually found in the seborrheic areas (front of the scalp)
The flakes stick to the scalp and to the hair to form sheets.