Land units are the bulk of almost every army. Your ACU, engineers, and primary units are land units. Land units have the lowest cost, and lowest build time compared to the other unit types. Ground units are used in probably 90% of the maps in Forged Alliance or the original Supreme Commander. They work well in numbers and can mount some serious firepower. They are also the only units that can be loaded in transports. Some units have abilities like hover, amphibious, radar, and engineering suite.
For a complete list of land units, please see the Land Units Category.