A ground attack is done by beaming colonists down to a planet that is owned by an enemy.
By default the Lizards have a 20:1 ground attack advantage. This means 1 Lizard colonist will kill 20 normal colonists on a planet with no defence outposts.
WARNING:
If a planet is left with 0 colonists any starbase in orbit will be lost. Any buildings on the surface of the planet will disappear slowly at a decay rate set by the host. If you wish to give a planet to another race you must leave at least 1 clan on the surface and have them beam their clans down right on top of you.