There are 2 types of Global Defense: Zombie War Upgrades. The first ones are the upgrades that require points earned from battle and are found in the Armory. Each of these skills have a limit to how high you can increase them and the cost of increasing them will go up with each upgrade. The second are upgrades in battle that increase a turrets range and attack or special effect which can be done twice for each unit.
More info at the Global Defense: Zombie War Guide Wiki
Attack Power – This will increase the damage output of a particular turret.
Combat Radius – This will increase the reach of a turrets attack or special effect in battle. 15 Distance upgrade = 1/4 square range increase on the battle field.
Flame Radius – Lengthens the reach of the Flamethrower so you can kill zombies from further away.
Sight – Increases the range of Radar during missions.
Freezing Effect (%) – Strengthens the freezing effect of the Frost turret so it slow enemies more.
Track Sum – This is for the Radar, but I’m not really sure what it does yet. It looks important though.
Build Sum Limit – I think this is suppose to be the number of zombies the Shockwave turret will slow down, but for me it doesn’t seem to work that way. Maybe it’s the pulse speed Im not really thinking so though.
Cooling Time – This reduces the cooldown time for the Space Laser, Shockwave Bomb, and Carpet-Bombing.
Turret Armor Enhance – This will raise your all turrets defense from exploding zombie and falling flying objects. Around level 40 your turrets will die in 1 hit so investing in this will be a waste.
Auto-Repair Per Sec – This will heal your units over time when they take damage. Around level 40 your turrets will die in 1 hit so investing in this will be a waste though raising this just a little bit will help in earlier levels.
Extras Lives – These will increase the amount of live you have in battle. Dropping below 20 lives will reduce the amount of stars you earn after a fight. So having more than 20 will allow you to get max stars when some zombies get by you.
Extra Money – This increase your starting cash so you can place the more expensive units right away.
Get More Upgrade PTS (%) – Probably your most important skill as it raises the amount of point you receive after battle. Level 3 Saudi Arabia on Hell is the most efficient place to farm for points.
Get More Money (%) – This will raise the amount of money you receive in battle from killing monsters and gold bars that are dropped on the field.