Brakes

 
Cantilever:

Cantilever, direct-pull, and linear-pull brakes.

These have each arm attached to a separate pivot point on one side of the seat stay or fork just below the rim. This solves the problem for standard callipers on wide tyres (such as those on mountain bikes) where the long distance from the pivot to the pad allows the arms to flex, reducing braking effectiveness.

The traditional cantilever has an L-shaped arm protruding outwards on each side, with a cable stop on the frame or fork to hold the cable housing and a straddle cable between the arms similar to the centre-pull brake.

The cable from the brake handle pulls upwards on the straddle cable, causing the brake arms to rotate up and inward and squeezing the rim between the brake pads.

Caliper

These consist of two curved arms that cross at a pivot above the wheel and hold the brake pads on opposite sides of the rim. These arms have extensions on one side, one attached to the cable, the other to the cable housing. When the brake lever is squeezed, the arms move together and the brake pads squeeze the rim. These brakes are simple and effective for relatively narrow tires, but have serious disadvantages if made big enough to fit wide tires. Low-quality varieties also tend to rotate to one side during actuation and to stay there, so that one brake pad continually rubs the rim even when the brake is released.

Centre-pull  brakes

These have symmetrical arms and by design do not rub the rim when they are released, by actuating the brake arms symmetrically. The cable housing attaches to a fixed cable stop attached to the frame, and the inner cable attaches to a sliding piece or a small pulley, over which runs a straddle cable connecting the two brake arms. Tension on the cable is evenly distributed to the two arms, preventing the brake from taking a "set" to one side or the other.

V-type

Linear-pull brakes (also known by the trademarked term "V-brakes").
These mount similarly to cantilever, direct-pull, and linear-pull brakes, but the arms extend straight up, and the housing is attached to one arm and the cable to the other, similar to the cable attachment for side-pull brakes. They are generally more powerful and easier to adjust than cantilever brakes but require a smaller gap between the brake pad and the rim surface. They function well with the suspension systems found on many mountain bikes because they do not require a separate cable stop on the frame or fork. Due to their higher mechanical advantage, linear-pull brakes require levers with longer cable travel than levers intended for caliper brakes or traditional cantilever brakes. 

Disc

Disc brakes consist of a metal disc attached to the wheel hub that rotates with the wheel. Calipers are attached to the frame or fork along with pads that squeeze together on the disc. Such brakes have been successfully used on motorcycles for decades, and are the principal choice there. Recent material advances in weight, costs and reliability have led several firms to develop and implement disc brake systems, and those are becoming a standard feature on many bicycles. They are used mainly on mountain bikes ridden off-road, but sometimes on hybrid bicycles and touring bicycles. Many tandem bicycles have a disc brake on the rear wheel in addition to rim brakes; the disc brake can be set to provide a constant drag, so that during long descents, the rim brakes are not overworked by the heavier machine.

Whichever type of brake you use a simple 30 second check is all that is needed.
 
First, check the amount of pad material that is in contact with the rim/disc, if it is worn - replace it!
 
Second, make sure that the brake blocks are in contact with the rim only, not rubbing on the tyre.
 
Third, have a look at the cables, if the cables are worn, frayed or rusty they will need replacement.
 
Overall brakes are an essential safety item, if you are in doubt consult a professional bike mechanic.