Shrubs are perennial, woody plants, usually with several stems branching out from the base, and smaller than 3 to 4 meters in height.  Because of their colonizing tendency, shrubs make up the main part of undergrowth, especially in clearings and wood fringes. They can form entire communities on the edge of the forest, which is sometimes a step leading towards more evolved ecosystems within the process of ecological succession (often resulting from the regression of an original forest that suffered severe trauma caused by natural or anthropogenic disturbances) or because of the particular environmental conditions beyond the timberline.

Shrubs are generally very long-lived, slow-growing plants with heavy hard wood. Often the underground parts are more developed than the aerial ones, and this characteristic, along with specific morphological and physiological adaptations, makes shrubs particularly resistant to harsh environmental conditions. Their often prostrate bearing, adventitious rooting ability, and suckering aptitude allow many shrubs to survive and to reproduce even after undergoing serious damage (fires, avalanches, landslides, trampling, grazing).

Their abundant and constant production of easily accessible fruits with high nutritional values is a valuable source of food for many animals. This contributes significantly to the formation of highly complex food webs that guarantee the stability and the efficiency of the ecosystems themselves.