The text below is quoted from the Game Trade Magazine July 2007 interview with Mike Elliot as posted on the Wizkids forums.
"Ground units combine with your space units to form your fleet. You can build a fleet with all ground
units, all space units, or a mix of the two types of units. The ground units have a tan colored base to
distinguish them from the space units. As a bonus, ground units get a +1 attack bonus during a strikes.
The drawback however, is that your ground units can attack only in your home zone - not in your opponents
home zone or the contested zone.
Don't feel that you are at a complete disadvantage away from home. Your ground units still attack back in any zone if they are attacked by a an opposing space unit. Generally, though, if you are marching your ground units forward, you are planning to strike your opponents objectives, and you can engage other ground units in any zone.
Because one of the goals of the game is to destroy objectives, the bonus that ground units get during strikes makes them a solid choice for decks that focus on destroying objectives instead of attempting to eliminate your opponents fleet. Ground Assault will feastures 20 new Objectives, including a number of unusual new abilities that will spawn new fleet and deck builds.
The set breakdown of Ground Assault is similar to the base set, with 67 units and 120 cards. Most of the deck types supported in the first set have some support units and cards this time around, and because there are several new icons, including Armor and Blaster, there are a number of new decks to be created as well."