The Goal of the game is to clear all 5 sectors. Each sector consists of several missions, and are visible within the map. The numbers on each mission indicate their difficulty level. Progress through all levels to unlock the next sector. Zombie's Often drop energy packlets. These are used as ammos(blue) and to recharge the armor(green). Your Armor and weapon can also auto recharge at a slower rate.
After each cleared mission you are able to scavenge the area. Level summary screen will show what you have found, usually credits and mod-chips. Mod-chips allow you to modify your weapon.