RTF rc airplanes are simply the best way to get started in radio control flying, particularly if you don't have any model building experience whatsoever.
The letters RTF stand for Ready To Fly which means that you, the consumer, don't have to do anything to the model to get it ready for flight, apart from install the radio control gear batteries (usually purchased separately) and do some very basic final assembly work, such as attaching the wing and tailplane to the fuselage.
The image below shows a typical electric RTF plane as purchased, in this case the popular Freedom Flyer from Megatech:

RTF rc airplanes have introduced thousands of people to the hobby of radio control flying, but it's only in recent years that they have become so widely available and cheap to buy.
Before RTFs were produced on such a grand scale, airplane kits were the way to go but these deterred potential modelers who just wanted to fly and weren't too interested in the building side of the hobby.
Nowadays, RTF models probably outnumber traditional assembly kits, although of course the traditional kits will never be completely replaced. There are countless modelers out there, myself included, who find building a model airplane to be equally as satisfying, if not more so, than the actual flying of one.
As previously mentioned, RTF rc airplanes often only need the radio control gear batteries to be purchased separately, but there are some manufacturers that offer a complete one box purchase, with the batteries included in the price of the model.
The Parkzone range of electric rc airplanes is an excellent example, and the planes are indeed hugely popular with beginner and experienced rc pilots alike.
Parkzone RTF planes are excellent quality, an example is the recently released Focke Wulf 190, shown below as unpacked from the box:

The two examples shown on this page are both electric, but of course RTF gas powered planes can be bought - in fact, the majority of electric and gas airplanes and helicopters that are sold by model shops today are Ready To Fly ones, which puts the popularity of these models into perspective.
An important tip to remember when choosing an RTF model is the availability of spare parts.
Buying a complete RTF plane for $30 on ebay, shipped from Hong Kong, might seem like a great idea at the time, but you might soon become unstuck when you need to fix it!
Ranges like Parkzone, or Megatech as another example, have a solid supply of original spare parts which should be available from the shop where you purchased the plane. If you are shopping around comparing products, always look at the availability of items such as wings, motors, tailplanes, props etc.
If you're seriously thinking of getting into radio control flying and don't want to build your model from plans, RTF rc airplanes offer you the best option of taking to the skies in the least amount of time, and with the least amount of fuss.
Always follow the manufacturer instruction manual included in the box, and you should have a safe introduction to rc flying with an RTF.
Red Rocket Hobby Shop has a large range of RTF rc airplanes.
ARF rc airplanes - a good introduction to model plane construction.
Plug n' Play rc airplanes - a great option for owning multiple models.
RTF rc helicopters - the fastest growing sector of rc flying