Those are most likely tapeworms.
What is going on is there is a tapeworm in your cat's intestine and what you are seeing is the tiny segments of tapeworm breaking off. That part of the tapeworm reproductive cycle.
To kill the tapeworm you need to get a medication called "droncit" or "praziquantel." I don't know if they have it at your pet stores so you will have to call and ask. You can order it here:
You cat got the tapeworm from eating an infected flea so you will need to treat your cat for fleas. The best products to use are "advantage" and "frontline" topical drops. I prefer "advantage" drops. You can get them from your vet or online.
You can read more about tapeworms here www.VeterinaryPartner.com/Content.plx?P=A&A=562