There are two different frontlines, one is frontline (available from vets, pharmacies, pet shops if qualified) the other is frontline combo which is only available from the vets. Frontline combo treats the external environment also. There is no difference to frontline from internet or shops.
With any flea treatment you need to keep to strict timelines, nothing over 4 weeks to treat otherwise the fleas may get a hold. Make sure you put it on two spots on the back of the neck and use the whole pipette, do not rub it in, do not let the cat go out for two hours after it has had it and if you have more than one cat/animal keep them apart for two hours whilst it spreads. If you are using frontline do not use any other products such as flea collars/other spot ons as the active ingredients in frontline will counteract the active ingredient in frontline.
You definately need to treat your house, fleas and eggs can live in the house for years, also think about treating your car if the cat went in it to the vets. Get a decent spray for the house, foggers and bombs only reach what the eye can see sprays can be directed where they need to be. Hoovering is the best thing you can so as the vibrations force the eggs to hatch, remember to treat the whole house, fleas can travel to rooms the animal doesn't go in by you.
Bear in mind the way frontline works is that fleas are attracted back onto the animal to die therefore you may still see fleas on the animal after treatment.
IMO in regards to worms (different poster) you need to treat for roundworms and tapeworms as a minimum, especially as tapeworms can be transferred by fleas, heartworm is not enough.