Answer:
Chlorine in water may be present in two forms, free and combined. Free chlorine does the hard work of killing bacteria and oxidizing contaminants. When you add chlorine to water, you are actually adding free chlorine. When the free chlorine combines with contaminants, it becomes combined chlorine, or chloramines. In water, this form of chlorine has very little sanitizing ability, and no oxidizing ability. Total chlorine is just the sum of both combined chlorine and free chlorine.
Which measurement you use, free or total, depends on your testing needs.