My pool has an ionizer attached just before the return. Does any one know what it does?

Adds metal ions, usually copper, zinc or silver to the water. It's not needed and can potentially cause pool or hair staining.

Thanks JohnT.

My parents have a pool with an Ionizer and my cousin is taking care of it this year. Pool store told him he needs to get ionizer cartridge replaced ($100 online). Can anyone advise the pros and cons of using heavy metals as an algaecide. I say buy bleach instead and save the money, seems simple to me. He may need a better explanation.

im sure chem geek will be around and explain it, but from what i have read the metal (copper) ions takes like 10 times longer to kill bacteria in the water than bleach does plus there are alot of other issues that copper does to the pool plus it can turn blonde hair green.....Mike

This is just general, someone with more experience will be here soon with a more detailed answer.
1) It is expensive. Those refill cartridges, like you said, cost a lot of money.
2) It does not kill bacteria/viruses fast. It is much slower than chlorine at killing viruses and bacteria, so will not prevent person to person transmission.
3) Chlorine is still needed. Because of 2 above, you still need to maintain chlorine in the pool. Since there is already chlorine in there, why not just use the levels we recommend here?
4) It is not needed. If you follow the recommended levels of chlorine, you do not need metals to prevent algae.
5) It can cause staining. With the levels of copper needed to prevent algae, you risk staining the pool. Copper stains are not easy to get rid of.

This post contains a table comparing the kill times for chlorine (with roughly our recommended FC and CYA relative levels) vs. copper and silver. As you can see, there is no comparison, and copper doesn't kill fecal bacteria. If one were to use metal ions for even controlling bacteria growth, then one would use both copper and silver combined, copper mostly for algae and for some bacteria and silver for the fecal bacteria, BUT there's no reason to do this because chlorine kills both quickly as well as viruses, etc. You also need a chemical to oxidize your bather waste and chlorine does that, while metal ions (i.e. copper or silver) don't.