There are 3 types of chlorine that you need to know:

Fixed or Free chlorine (FC). This is the stuff that does the killing of bacteria and algae.

Combined Chlorine (CC). This the chlorine in the form of Chloramines that contain all the bad stuff mentioned above coming from body sweat, sunscreen, and other badies that have not been destroyed yet. This is normally where the "funky smell" comes from.

Total Chlorine (TC). This is the sum of the first 2:

TC = FC + CC

The chlorine disappears from the pool via evaporation and ultra-violet rays from the sun. Stabilizer, (CYA) helps to keep the Cl from degrading too fast, and holds it until it is needed. That's why you need to have about 40 - 50 ppm for the chlorine not to be used up too quickly.

Hope this helps.

Pat