The Enemy: The Cat Flea
The flea that lives on cats is aptly named "The Cat Flea" and it is one of the most abundent and widespread fleas in the world. One of the most important reasons to use Frontline for your cat is because the cat flea can transmit other parasites and infections to your cat (and also to you.) The cat flea is the most prevalent flea on dogs as well so if you have a dog too you should of course make sure they have Frontline.
Picture of a Cat Flea larva:

Picture of Cat Flea fecal matter also known as "flea dirt"
Enemy #2: The Tick
While not quite as common as the Cat Flea, ticks are actually more dangerous to your cat's health than fleas. Ticks harbor many dangerous diseases for both your cat and also for humans.
Frontline works within 48 hours to kill all of the ticks on your cat. Order Frontline Plus For Cats From PetCo.com
Picture of a tick: