Can any one tell me about the use of drum, v brake and disc brakes... which is better and which can be used at front wheel and which is better to use at rear wheel for a cycle moving at max speeds of 25kmph?
disk brakes are generally considered the strongest and they work well in wet or dry conditions. But they are also more expensive and require special wheels, frames, forks. Drum brakes work well in wet and dry, but aren't that great overall. They are good for riders that do a lot of heavy breaking because they won't overheat the rim like v-brakes can. V-brakes work very well for most bikes. They do not stop as well in wet conditions and will wear rims over time. But are simple, cheap, effective.
All can be used for either wheel. Better breaking power is more important on the front. Rear brakes are not as critical. roughly 2/3 of your breaking power is on the front wheel.
Never thought to asked this question, is a coaster brake considered a drum brake?
Good maintenance to your Bike, can make it like the wheels are, true and smooth!
Simple solutions are always the best, and for SHORT reach brakes such as a road bike, caliper brakes are very effective.
Canti or v brakes are ok for general use on wider tyres requiring a longer reach, I wouldn't go for discs unless you are doing off road in wet conditions, and then cheap ones should be avoided.
If you're looking at a new bike, go for discs. My discs far out-performed my linear-pull brakes last winter in the snow, and did an amazing job during spring breakup when the roads and paths were filled with icy slush.