Dandruff is a chronic scalp condition, whereas buildup is conditional, caused by hair product usage and cleansing habits. Dandruff is caused by a yeast, which inflames the scalp, and is marked by scaling on the scalp that causes itching and flaking. Scalp buildup, on the other hand, occurs when hair is not cleansed adequately or often enough. While buildup typically causes itching and flaking, it does not have the scaling of dandruff and the make-up of the flakes is different.
The scalp, like the rest of your skin, is continually shedding cells. However, the rate of producing and shedding skin cells on the scalp is faster than on other parts of your body. Add to that, these shed cells are retained in the strands of the hair, making them more visible. Therefore, not washing for a week or two will create the appearance of dandruff in most people.