Deforestation is a term that, at it's simplest, means the destruction of a forest. It can apply to natural forces that destroy a system of trees, like a forest fire or a disease which kills off all the plant life in a given area. However, recently the term deforestation has come to apply to the casual destruction of a forest through artificial, human controlled means.
One of the most common reasons a forest is destroyed by man is for lumber. Wood is one of the most basic of commodities, and vital to the survival of man. This need can be taken to extremes, however, and some people strip all the trees in a given area and render them into lumber and wood by products like glue and paper. In cases like these, the term deforestation applies not only to the loss of the trees, but the forest's inability to cope with the loss by natural seed germination. In simple terms, the number of trees that get destroyed are at a higher and faster rate than the forest can grow now ones back.
Another common deforestation information that brings about its occurrence is the conversion of the land on which the trees are into arable farm land. With mankind's needs for food increasing with a burgeoning population, the land areas covered by forests are razed by builders who take over the land and either convert it into pasture lands for animal grazing or for farm lands for growing crops. Since the land is converted into something beneficient for man's needs, this type of deforestation is considered to be the most benign, especially if the cut trees are also used for lumber instead of simply discarded.
However, one form of deforestation for creating farm lands is highly frowned upon, both for it's sheer destructiveness and waste of resources. This is the use of fire to clear a forest away. In some third world countries, farmers who want to use a certain patch of land for crops or grazing fields will resort to fire as the quickest and simplest means of clearing away large tracts of forest. This is both a safety hazard and a waste, not only because the fire may rage out of control and consume the entire forest, but also because the trees, which might have been converted into useful lumber products, are simply destroyed outright. This also causes damage to the soil of the cleared land, so that crops grown there are substandard quality. Knowing this deforestation information will save not only forests but consequently, humans, too.
Another deforestation information is the reason for deforestation that can sometimes be a cause of controversy is the razing of a forest for urban renewal. This basically means that trees in an area get cut down so that the land itself may be used as a place to construct urban architecture like houses, apartment complexes, and buildings. The reason for the controversy is because of the potentially damaging effects the loss of a large number of trees will have on the environment. Compromises have been reached where such razed sites will often have their own parks where trees, grass, bushes, and other vegetation are planted, nurtured, and allowed to grow.
The last reason for deforestation is for mining and petroleum. Some forested areas have high concentrations of iron ore, mineral, and oil deposits which are valuable natural resources. Under normal, acceptable circumstances, the trees in the given area are rendered into lumber and used productively, and only when the area is clear of trees is the mining begun. However, in some areas, again notably third world countries, the "quick and dirty" solution of burning down the forest is used to clear the land and get to the mineral deposits as quickly as possible. This type of deforestation is a waste of what would otherwise have been productive lumber.
Aside from the waste of trees through fire-type deforestation, another reason it is frowned upon is it's impact on the environment. Forest systems have a huge effect on the atmosphere, not only by generating breathable air, but by controlling the amount of moisture that cycles in the air. Deforestation has been known to have an adverse effect on weather patterns. Another ecological effect of deforestation is flash floods. In some climates, forests take in the water from heavy rainfalls, so that little if any excess water reaches the lower landscapes. The sudden loss of trees in such areas gives the water from such rainfalls only one place to go - nearby settled human habitations. This results in heavy floods and similar disasters.
Lastly, it should be noted that forests support their own eco systems, with balanced flora and fauna. Deforestation has been known to render some species extinct or near-extinct simply by removing the places where these animals live or these plants grow. Given that some of the largest contributions to modern medicine have been made by studying rare plants and animals, it is also widely held that deforestation robs man of the potential benefits he might otherwise have discovered in the destroyed forests. If you don't take into heart the abovementioned deforestation information, it's almost like you've dug your own grave.