Ground defenses consist of three types, two of which are found under Ground Defense on your Shipyard page: AA's (Anti-Aircraft) and GML's (Guided Missile Launchers). A third type, civil defense, is one that you do not have to build yourself.

It is generally considered foolhardy not to build a good number of these at your colonies and/or capital city. A good set of ground defenses can keep the enemy at bay while a defense fleet rushes to your aid. Without them, your colony will fall like a house of cards. Checkmate.