If you were effective enough to hit somebody with your attack, it is not guaranteed that you actually hit and damaged him yet. Your opponent may still try to parry, block or evade your hit. But it will cost him his action for the round.

If you defend against an attack, you must always roll a d10, and add a certain skill or ability. Your defense is succesful if you make at least the same number as the attacker made with his attack. So if you are hit with a 12, you must make at least a 12 too. A natural is always enough to defend. If you fail your defense you are hit by the attack, and probably you will be wounded or even killed. How bad the wound will be depends on your armour, and how badly you messed up your defense. But first, here are the different ways to defend:

Evasion is dodging or ducking the blow. To do so, you must roll d10 and add your DEX. Evasion can normally not be used in combination with an attack. But if you roll a natural for your evade, you not only evade, but may attack back in the same round too.

Blocking is done with a shield, or a left-handed weapon. Roll a d10 and add your shield skill. You can block with your shield and attack with the weapon in your other hand in the same round if your block is succesful.

Parrying is done with your attack weapon, catching the blow of the enemy. To parry, you roll a d10 and add your weapon skill. If you roll a natural for the parry, you may attack back in the same round.

You can defend only once against any attack (aimed or not). The only exception is when you decide beforehand to block with both your weapons, or both your shield and weapon.

Machteld faced her enemies with dread. Ska warriors. Two of them at the same time. And they were fast (the two Ska have the initiative and go first, Machteld will go last). The first one struck her with his fierce blade, and she only just managed to throw her shield in between (the Ska rolls 6+4=10 for his attack, Machteld defends with 8+3=11, one point more). The second one attacked swiftly then, but barely missed her (4+4=8 is a miss). That gave Machteld the opening to use her own sword, and she thrust for his heart (9+4=12, chest). He jumped back far, and she missed. But now she had the edge on him (the second Ska already attacked this round, and so borrows his defense from his next round; he rolls a 9, plus DEX 3; he evades, but in the next round he will go last).
The first Ska left Machteld no breathing space. He hit again, now damaging her shoulder ... (the new round begins, with the first Ska still having first initiative, Machteld being second, and the second Ska now being last).