There are three types of chlorine for pools that can be used in swimming pools: free chlorine, chloramine, and oxidizer.
Free chlorine is the most common type and is simply a chemical that breaks down into hydrogen chloride and oxygen. This is the type of chlorine most commonly found in household bleach, pool filters, and sanitizers. It is effective at killing bacteria and viruses but does not work well against algae.
Chloramine is a mixture of free chlorine and ammonia-based disinfectants. It has been shown to be more effective than free chlorine at killing bacteria, but it can also cause eye irritation and skin rash if exposed to too much of it.
The oxidizer is a harsher form of chlorine that destroys both bacteria and algae. Each type of chlorine has its own advantages and disadvantages, so you need to do your research.