What do the servos control?
There are usually five servos on an R/C heli. One controls throttle, one controls collective, one controls fore-aft cyclic (analogous to elevator), one controls left-right cyclic (analogous to aileron), and one controls tail rotor pitch (analogous to rudder).

What is the use of gyros and how do they help?
The gyro is positioned so that it senses yaw. It then feeds small inputs to the tail rotor servo to counter the yaw that it detects. This keeps the helicopter from yawing to the left and right when you don't want it to. Left-right movement of the left stick also supplies input to the tail rotor servo; so you and the gyro are both giving control inputs to the tail. A gyro is a MUST. It's probably not an exaggeration to say that gyro-based stabilization of the tail rotor made R/C heli flying feasible. It is possible to fly an R/C heli without a gyro, and it's also possible to juggle seven balls. It's just darn hard! Furthermore, it's definitely not something you want to try tackling when you're just getting started. Without a gyro, the heli can begin to whip around wildly as soon as the skids leave the ground. The heli will do a 180-degree turn and you're looking at an angry helicopter coming right at you before you know what happened. Definitely not something for a beginner to tackle.

How about fixed-pitch versus collective helis?
Helicopters with collective are now inexpensive and reliable. Every reasonable modern heli, from beginner-trainers up to FAI world-beaters, has collective. In a fixed-pitch heli, lift is controlled by varying engine RPM, just as in an airplane. This is an outmoded technology, and you will outgrow such a heli very soon. Virtually no aerobatics, no auto-rotation (if the engine quits at altitude, the heli becomes a brick), not as much fun.
Radios

How many channels do you need to control a heli and why?
You need five channels to control a heli. You need one each to control pitch, roll, and yaw. You need one to control throttle, and you need one to control collective. You might think that one servo could control both throttle and collective, since they are related.

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