If your car veers to one side when you press the brakes, the braking force is uneven from side to side. It is both annoying and a safety issue. Here is what causes a car to pull when braking.
A sticking caliper
The most common cause is a caliper that is not applying evenly - a stuck or lazy caliper on one side brakes weaker (or drags), so the car pulls toward the stronger side; see sticking caliper. A dragging caliper also runs that wheel hot; see why brakes get hot.
Uneven pads or rotors
Mismatched pad wear, a glazed pad on one side, or a badly scored rotor makes one wheel grip differently than the other, causing a pull; see brake wear. This is why pads and rotors are replaced in axle pairs.
Not always the brakes
A pull only under braking is usually a brake problem, but a collapsed brake hose (acting as a one-way valve), a seized slide pin, or even a suspension or alignment issue can contribute; see caliper bracket. A pull all the time is more likely alignment.
How to fix
Have the calipers, slides, hoses, pads, and rotors inspected side to side. Free or replace the sticking caliper and service both sides evenly; see brake drag. Fit quality pads and hardware.