The skill.cfg file defines the amount of health, the weapon strength of a monster, and the player's weapon strengths. It is located within the .pak file so you'll need a tool like Pak Explorer to get to it.
Following is an excerpt from the skill.cfg file.
The first three lines define the Headcrab's strength. The last three define how much damage it does to you. Why are there three definitions? There's three because there are three skill levels. When the skill level is set to easy, the Headcrab's health will be 10 by the definition of sk_headcrab_health1. 2 would be Medium and 3 would be Hard. The same goes for it's damage. Two slashes ( // ) define a comment. That's pretty much all there is to it.
Now you can't just go in and add a new definitions to a monster. You can only change the amounts. Also note that at the top of skill.cfg file it says: "DON'T MESS WITH THIS." Edit this file at your own risk.