A style of rigging where instead of supporting things by motors or a fly system from over head everything that is off the ground is supported from the ground by towers. These are often very complex truss structures designed for the intended purpose only. Light weight systems can be set on casters and moved to fit the need but heavy lifting systems often have very large feet that support very heavy towers and require a significant part of a work day (or more) to move or modify.
Systems basically break down to mobile and static systems.
For one manufactures breakdown of important pieces look at Xtreme Structures and Fabrications page on ground support (Note: The tower that is however tall is necessary goes between the four foot section pictured and the head block, basically break the spigots right by the number 5 and add truss as necessary)