From Envirowiki

Logging is the process of cutting down trees (turning them into logs), for timber, woodchips/fibre or firewood. In the past, this was done with axes, then hand saws, and snigged out with bullocks, but in the last century or so, chainsaws and bulldozers and trucks have allowed logging to the point where it has become clear-felling, removing whole sections of forest. This is so entrenched in the system that "selective logging" now means clearing most of the forest, and selecting which bits to leave standing. This is especially so when the timber is required for woodchip, or firewood, where wood quality doesn't matter much.

[edit] impact

Clearing forests destroys ecosystems, and releases large amounts of greenhouse gasses, as the dead life rots, and drying soils release methane. Tropical forest clearing is one of the worst on this count, as the forests have large amounts of hummus and organic matter on the ground.