From Plarium Games Wiki
Troops protect your Sector, invade other Sectors, and help your Friends out when they need it. Research Technologies and construct new Buildings to unlock new, more powerful Troops.
Offense and Defense
Troops can be either Offensive and Defensive (with the exception of Cyborgs which have the same value for both). Click on the unit when purchasing it to see detailed stats. A Red border indicates Offensive Troops, a Green one means they're defensive. Defensive units are essentially useless when attacking but highly effective at protecting a Sector or Deposit. Conversely, Offensive units are designed for attacks but have a low defense value. If your Sector has tons of Offense and no Defense - don't be surprised when you get Invaded. A good tactic is to develop dedicated forces of either Offensive or Defensive units tasked with either taking or holding Sectors.
Regular vs. Elite Units
There are two variants of every unit in the game - Regular and Elite. Regular units are purchased with Uranium, Titanium and Credits. Elite units are purchased with Crystals and cost significantly less Resources. Regular and Elite units are otherwise identical in performance and stats.
Your military is divided into four different branches: Infantry, Armored Corps, Artillery, and Aviation. Each one of these Branches fields 4 different units - 2 Offensive and 2 Defensive. Each of these units have their own particular balance of effectiveness against Offensive or Defensive units of each branch. Click the question mark or directly on the image of the unit to find out detailed information about its strengths and weakness.
Using your Troops
All combat units are produced by specific Military buildings. Once purchased, you can either use them yourself or they can be sent to Allies as Reinforcements. Go to the Command Center and open the "Reinforcements" tab to send them to your friends. Reinforcements sent by Allies cannot be used for offense - only for defense. Research new Technologies to unlock new types of units. The higher the level of Technology, the stronger the unit will be during an engagement.