I've been meaning to post this for a while. A lot of people mix up three terms in GURPS as if they were one - basic damage, penetrating damage, and injury. I do this sometimes, too, using "damage" to mean all three. Worse yet, I also toss in "damage past DR" as if it was a term of art and then adding in "HPs of damage" or "hit points lost" or "hit points inflicted." The trouble is that if you go reading the rules, you end up with questions that the correct terms answer in and of themselves. Like, does "triple damage" mean before or after DR? If a blow does "half damage" what are you halving, the roll or the damage? Is it before or after wound modifiers that you determine crippling? The rule says "take 1d-3 injury" or "lose 1 HP" - does my DR count? And so on.
This is all out of GURPS Basic Set: Campaigns, p. 377-381.
Basic damage is the result of your damage roll.
Penetrating damage is your basic damage minus any DR that applies to that attack.
Injury is penetrating damage after you apply any wounding modifiers.
Here are a couple examples:
Otto (ST 15) swings his Broadsword and hits a normal human foe wearing mail armor (DR 4, 2 versus crushing) in the torso. His damage roll is 2d+2 cutting; he rolls two ones, getting a total of 2 + 2 = 4 basic damage. His foe's DR 4 reduces that to 0, so he has 0 penetrating damage and thus inflicts 0 injury.
He swings again and hits again, and this time rolls a six and a three, getting a total of 9+2 = 11 basic damage. His foe's DR 4 reduces this to 7 penetrating damage. The multiplier for a cutting weapon against the torso is 1.5x; 7 x 1.5 = 10.5, which rounds down (per p. B379) to 10 injury. That's how many HP his foe loses.
If his foe got back up as a zombie (with Injury Tolerance: Unliving) and Otto foolishly chose to stab him, it wouldn't work out as well. Let's see how that goes:
Otto hits his re-risen foe with a stab to the torso. His damage roll for a thrust is 1d+3 impaling. He rolls a four, getting a total of 4 + 3 = 7 basic damage. Sadly, the mail of the zombie is still worth DR 4, and Otto's 7 basic damage becomes 7 - 4 = 3 penetrating damage. The zombie's Injury Tolerance makes the wounding modifier for impaling only x1, so Otto only inflicts 3 injury.
Going back to those questions, it's pretty clear what they intend. Triple damage critical? It triples basic damage; make that damage roll and multiply it times three. Half damage? Roll the damage roll and halve it. Crippling is after wounding modifiers, since only injury is actual HP loss. And if something says "take injury" or "lose 1 HP," it just comes off your HP, since DR protects against damage, not injury.