Troops move and fight in a tight formation of one or more ranks. Such a formation is often referred to by an appropriate title such as a regiment or, in the case of cavalry, a squadron, or it can simply be called by the cover-all term — unit.
When a unit moves around the battlefield it must maintain its formation, which means that models are not free to wander off on their own. The formation can move straight forward as a body perfectly easily, but if it wishes to change direction then it must make a manoeuvre.
There are three specific types of manoeuvres that enable a unit to turn about or rearrange its ranks: wheel, turn and change formation. In addition there is a fourth special manoeuvre called reforming.