What is deforestation?
Deforestation is when humans remove or clear large areas of forest lands and related ecosystems for non-forest use. These include clearing for farming purposes, ranching and urban use. In these cases, trees are never re-planted.
Since the industrial age, about half of world's original forests have been destroyed and millions of animals and living things have been endangered. Despite the improvements in education, information and general awareness of the importance of forests, deforestation has not reduced much, and there are still many more communities and individuals who still destroy forest lands for personal gains.
Why do humans clear forest lands?
Trees are cut down (deforestation) for many reasons including
To be used, sold or exported as timber, wood or fuel (charcoal). This is called logging.
To be used for farming purposes (grazing fields for livestock, or large scale farming activities)
To make room for human settlement and urbanization (these include making space for shelter, industries and roads)
To make room for mining. (More on deforestation here)
In all the reasons above, the trees cut are usually very well developed trees that have taken many years to mature. When they are cut, they break down many more younger trees as they fall to the ground, leaving that area heavily degraded.
Forest degradation is different from deforestation. Degradation is used to mean the destruction or reduction in quality of specific aspects of forests. Example, a decrease in tree cover, changes in their structure or a reduction in the number of species that can be found there. If Acid rain destroys trees in a vast area, it can be called forest degradation.