What should I buy? You should buy a Futaba 9C Super or a JR 9XII, or even a Spectrum DX7.

Transmitter
The transmitter is the control box that you use to command the helicopter. Nowadays these have become very comprehensive computer controlled devices.
You set up a lot of how your helicopter performs via the transmitter.
Ideally, you want a minimum of 8 channels for your transmitter, but you can use 6 channels if need be. I recommend getting at least an 8 channel radio, not so much for the number of channels, but for the extra added features you get with these radios.
There is a lot of debate about what kind of transmitting system is best, either PCM, or PPM. My opinion is that you should get the PCM system because you can always use PPM on it if need be. You can't do it the other way around. Plus, some helicopters, like gassers, require PCM modulation. Remember you're planning for the future here, you never know, you might wanna get a gasser later on!

Transmitters I like:

• Futaba: 8U Super, 9C, 9C Super, 9Z (WC, WCII), 14MZ
• JR: 3810, 9X, 9XII, 10(X, SX, etc)

Transmitters I wouldn't recommend:

• Hitec ones (User interface is terrible and there isn't a whole lot of support around for them, plus they feel cheap and tacky).

Transmitters I haven't played with but have heard good things about:

• Airtronics (Alan Szabo Jr flies them so they can't be bad - they're just not popular)
• Spectrum DX7 (these things are starting to show up everywhere, they're the future of R/C that's for sure)
• Multiplex: We don't see a lot of these round here, but they're big in Europe

The more popular makes are JR and Futaba. Both make excellent quality products. I recommend getting either the JR 9XII, or the Futaba 9C Super. If you really want to splash out, get the JR PCM10x, or the Futaba 14MZ, but you're only buying these cause you like flashy things. You won't fly better because you have them.

Mode One or Two (or three or four)?:

There are two popular types of control layouts with transmitters. They are Mode One and Mode Two. This refers to which sticks look after the various controls of the helicopter.
Mode one is when the left stick controls the fore/aft cyclic and the rudder controls. The right stick controls the collective/throttle and left/right cyclic.
Mode two is when the left stick controls the collective/throttle and rudder controls. The right stick controls the left/right cyclic and the fore/aft cyclic.
In the US, Mode Two is the more popular layout, however in Australia, some parts of Europe and most of Asia, Mode One is more prevailent. I would generally recommend starting on Mode Two, however, it is important to get the same layout that the people who will be teaching you use. That way they can fly your machine if need be. Don't go learning Mode Two just cause you think Piroflips will be easier, yes they might be, but you'll also have a harder time doing other maneuvers that Mode One people will find easier...

Receiver
If you purchase a new radio set, you'll most likely get the corresponding receiver with it too. The receiver just act's upon the signals the transmitter produces and commands the appropriate servos/gyro etc accordingly.

There's not really much more to say about receivers.

Servos
Servos are the little gadgets that actually do the work in the helicopter. They move the little arms that make your helicopter move.
You can get all types of different servos. The difference is mainly in the speed of the servo (transit time) and the amount of power it produces (torque).
When you're first beginning to learn and you're not thrashing the helicopter round, basic servos that come with your radio kit (such as Futaba 3001's etc) will be fine cause you're not putting them under large loads. However, when you start throwing your machine in around, or if you upgrade to a larger machine such as a 60 or a gasser, you'll want to upgrade your servos to a higher torque.
There are also two types of servo, the Digital servo and the non-digital. Digital's are designed to center more accurately and also come to full power from stop. Non-digitals are less expensive, but some do not center as accurately, and also don't have full strength in minute movements.

Buy the best servos you can afford, putting cheap servos on a good helicopter is like putting bald tyres on a nice car, it just won't go as well. These days I would always recommend getting digital servos if you can swing it, they're just a LOT better than analogue.

In general, servos from Futaba, JR and Hitec are the most popular, Airtronics have a good following too cause they have some mega servos.

Batteries
Your radio set will often come with a battery for the electronics in the helicopter. A lot of them are around 1000mah. These are fine, but you might want to consider upgrading to a larger capacity in the future. Some gyros and servos really chew up battery time. My F3C machines have 2800 Nicad packs and I generally do about five or six flights before I recharge those. The 3D models both have regulated systems (Duralite and Fromeco) and have at least 4000mah batteries in them which means I can go for AGES between charges.

What is really important here is to make sure you have a QUALITY switch, not a small nasty cheap one. Get the Futaba Heavy duty one, that allows a lot of current to flow. You'd be amazed how much of a difference a good battery and switch makes.