Features
Frame
The bike frame is made either with attention to reduce weight (aluminium frames, without shock absorbers) or to increase sturdiness (as in case of mountain bikes).
Handle-bar and seat
Relative orientation and posture
The seat and handle-bar height and angle, to a large extant determine posture while cycling. They determine what portion of the weight is borne by the arms, wrists, back, bottom etc.. The seat height also determines the extant to which the leg straightens while pedaling.
Handle bar supports
The handle bar may provide various provisions for supporting the weight of the body - including elbow pads, wrist-grips etc..
Seat softness
Softer seats provide more comfort and cushioning - though they are liable to absorbing water in a rain. Tieable seat covers made of materials like gel or memory foam may be used for additional softness.
Wheel
Important features include circumference, truing - the closeness to being a perfect circle. Bikes sold in supermarkets often undergo very rough handling, causing the truing to be defective.
Double-walled rims can undergo great punishment without loosing truing.
The wheel encloses the tube. Around the rim is a strip of rubber used to ensure that the spokes don’t poke the tube.
Axle
The ‘axle’ which holds the wheel should ideally have as little friction as possible.
Tire
Tire breadth determines grip.
Brakes
Brush brakes tend to be less effective in rainy season than disk brakes. The latter are also less affected by loss of truing.
Brake-application
Brake-application has several interfaces. Most common is a pair of levers on either side of the handle-bar. In USA, the left lever controls the front brake and the right lever controls the back brake. In Europe and Asia, the reverse is true.
Brake is sometimes applied using a mechanism so that pedaling backwards applies the brake.
Gear
Gear size and rotation speed
Gears employed affect the speed with which the cycle chain completes a rotation. Use of larger gears attached to the pedal increase this rate. Use of smaller gears attached to the rear wheel increase the rate at which the rear wheel turns relative to the chain.
Gear shifting
Gear shifting is done using either the grip-shift or the lever shift mechanism. There is usually one mechanism for the front and the rear gears - on the left and right handles respectively in USA. In US bikes gear shifting only works while pedaling.
Gear shifting happens by shifting the chain laterally. In case of some bikes, shifting into certain combination of front and rear gears results in the chain brushing against the shifting mechanism - this is resolved by shifting the gear appropriately. Some bikes avoid this by making those combinations impossible/ ineffective.
Categories
A road bike is often lighter frame (making carrying easier), with a larger wheel to allow for traveling greater distance with lesser pedaling, and with a very thin tire inflated to such a high level that often only a thin line touches the ground (thereby reducing friction).
A mountain bike has a sturdier frame, with good shock absorption abilities, with relatively smaller wheels, a broader tire.
Hybrids combine mountain bikes and road bikes. They have a broader wheel while maintaining a thin tread on road due to high inflation.
Purchasing
Assembly quality
When buying a non-lower-end bike, it is best to visit a shop where they are assembled by a good mechanic.
Supermarkets like walmart employ low-payed, low-trained mechanics to assemble bikes. They do a less thorough job. Eg: Lost truing results in repeated tire distensions, gears don’t function properly.
Usage
Used Bikes certified and inspected by mechanics are better.