DarrkPhoenix: In battles against human units (i.e. not mindworms) each unit has a set attack and defense value (a unit's stats are primarily expressed by three numbers- attack, defense, and movement). If you're attacking you want to use units with high attack, while if you're defending you want units with high defense. Most of the default units have either a high attack or a high defense, but not both (you can make custom units with both, but they're quite expensive to produce). This means you need to take note of what a unit is suited for and use units for appropriate roles. A 1-10-1 unit will get creamed if you try to attack with it, while a stack of 12-1-3 tanks will be easy pickings if your opponent attacks them first. There are also various things that modify those base attack and defense values- tile defense (including base defensive structures), moral (whether a unit is hardened/veteran/elite), and any special abilities the unit was constructed with (e.g. a blink displacer will allow a unit to ignore defensive bonuses from base defenses).

Psi combat (mainly used by mindworms, but also a possible unit upgrade late game) is a whole different matter. In psi combat standard attack and defense strengths are ignored; instead attackers are given a base strength of 3, defenders a base strength of 2, then certain bonuses (such as moral) are applied. Many other bonuses used in regular combat are also ignored (e.g. base defenses).