Hedges provide a nice green soft way of providing privacy or visually blocking unwanted views and making green fences.
There are two basic types of hedges: large shrubs that grow dense and compact, and some trees that respond well to topping (a "no no" for most trees). These trees generate new growth from below when pruned, developing a thick mass of compact vegetation.
Tree type hedges generally grow faster and taller than shrub hedges. This can be an advantage but they also need to be pruned more often, and if not topped regularly they can become sparse.
The first rule of pruning is to understand that when a plant is pruned in one spot it grows in another. By topping the hedge the plant sprouts growth below. By pruning the sides we keep it dense.