Every weapon in Evolution RTS is given a damage boost to one armor type. In the unit description, it is shown what type of armor the unit has, and how much of a damage bonus that unit has vs another armor type.
In this screenshot we see several different vital pieces of information.
First of all, the larger tank is selected, and notice that the mouse cursor is hovered over the green enemy tank. Notice that above the Unit Tooltip that it says 100% damage dealt and 200% damage received. That means that the selected tank (the large red one) will deal 100% of it's base damage to the smaller tank, but will suffer 200% base damage from the smaller tank. As the mouse cursor is hovered above the smaller tank, we can see it;'s description.
It is a Light armor class with a 200% damage boost vs armored units.
From this we can quickly deduce what units counter what. In this particular situation, it's a little tricky because the large tank is an armored tank destroyer with a damage penalty vs buildings, whereas the smaller unit is a light tank destroyer with a damage bonus vs armored units.
The fact that the larger tank has a damage penalty vs buildings denotes that it is good vs light and armored units, but not good vs buildings, so in effect, these units counter each other, albeit indirectly.
As with all RTS games, you must learn the units and their counters. For some this might be somewhat more difficult considering the amount of units available, but you will find that after a few games it becomes pretty clear.