From someone who has played 2nd edition. (I am not 100% sure, but I would assume that 3rd edition would be the same) You have your Defense score and then you have a Toughness roll. When someone makes an attack on your character you compare the attack roll to your defense, if it meets or beats it, it hits you. Then with every attack there is a Damage modifier.
So lets say we have Major Glory fighting the evil Death Dealer. The Death Dealer has a playing card deck that he uses as his weapon which has an attack of +10. Major Glory's Defense is 20. The attack is rolled and Major Glory is hit.
So now that the character hit we look at the damage which is determined by the power he used. So Death Dealer used his Blast power which is a Level of 10, so it deals 10 damage. Now the damage will be a DC in which the defender has to roll against with his Toughness save. A Damage DC is always 15 + damage modifier, in this case 10. So, the DC in which Major Glory has to meet or beat is a 25. Major Glory has a Toughness save of +10 so he makes his roll. Rolling a 15 or better means nothing happens, it didn't hurt him that bad, he was able to take it. Now if Major Glory were to roll a 10-14 (thus making it a 20-24) he would take one Bruise. If he gains a bruise all following toughness rolls are rolled at a -1 and for each additional bruise he continues to accrue another -1s so eventually it will become harder and harder to beat the damage DC.
Now if he were to roll a 5-9 on his first roll or get a 15-19 he would not only take a bruise, but will also be stunned. Stunned means that he will not be able to do anything on his turn this round and a few more thing, but I can't quite remember all of them off-hand.
Finally if he were to roll a 4 or below on his first roll or get a 14 and lower he would be knocked on conscious and be down for the count.
And that is just an overview on how damage works in this system. It really does work out really well. And while being knocked out first round can happen it is not very likely to happen because of something called hero points, so no worries on that end. And there is more to how damage works, but that is how it works on a basic level.
I like to think of it as kind of doing damage backwards, instead of rolling numbers to lower the person's HP the defender is making a roll to block the incoming damage and depending on how well the roll is means how much damage they will take.
I hope this helps.