Combination skin is simply having oily skin in some areas of your face and dry skin in other areas. Typically, an oily T-Zone (forehead, nose and chin) and dryer cheeks indicate the so-called combination skin.
The oilier parts of combination skin are caused by an overproduction of sebum. The drier parts of combination skin are caused by a lack of sebum and a corresponding lipid deficiency.
WHAT CAN YOU DO? Treat the different parts of your skin differently. Use products for oily skin in the oilier areas and products for dry skin on the dry areas of your skin.