A decrease in area covered by a forest is the definition of deforestation. Deforestation may be accompanied by regeneration or it may occur without regeneration (FOA, UN). It is important to not that deforestation, which may be accompanied by forest regeneration, is not the same as forest degradation, which indicates decrease in the quality (health) of forestation, not area of forestation.
The forest area is constantly decreasing, especially in the developing and underdeveloped countries. Some of the key causes of this include, need for agricultural space, wood, overgrazing, urbanization, etc. In our quest for higher agricultural production, to satisfy our ever-increasing population, we are exploiting forest area and converting it to agricultural lands. This is especially true for heavily dense countries, esp. in Asia and Africa. Our demand for wood is constantly increasing and a cheap source of wood are the forests. In several areas, cattle consumes large portion of forest plants and thus destroys the cover. We also are rapidly expanding to accommodate our population and are acquiring more and more land. This addition land comes from forests. We also obtain a large number of other resources from forests, including, animals, wood, fruits, etc.
Thus, to fulfill our needs, we are converting more and more forest cover to agricultural or urbanized lands, thus seriously damaging our forest cover.
Hope this helps.