Flea Collars are pretty effective for cats as well as dogs as long as the cat or dog doesn't have a flea infestation.
First you need to give the cat or dog a flea bath to kill off any fleas or flea eggs so that the majority of the fleas are gone.
Then put the flea collar on the cat or dog and it will help kill any fleas around it's neck and prevent further infestation of fleas.
You still need to give the cat or dog a flea bath often to keep fleas away from your dog or cat but the flea collars does help.