When you pull a set of worn brake pads, do not just toss them - the way they wore is a free diagnosis. Even wear means a healthy system; uneven wear points to a specific fault you should fix before installing new pads, or the new ones will wear the same way. Here is how to read the pattern.
Even wear across all pads
If the inner and outer pads on each wheel are worn evenly and the two sides match, your calipers, slides, and hardware are doing their job. This is the goal. Just confirm the pads are not simply worn out - see when to replace brake pads - and install fresh ones with new hardware.
Tapered wear (one end thicker)
A pad thicker at one end than the other - wedge-shaped - usually means the caliper is not sliding freely on its guide pins, so the pad cocks against the rotor instead of clamping flat. Clean and re-grease the slide pins and replace worn hardware. Tapered wear left unfixed also chews rotors unevenly and can cause a warped-feeling pedal - see warped rotor causes.
Inner pad worn far more than outer
When the inner pad is nearly gone but the outer still has meat, the caliper piston is likely sticking - it pushes the inner pad hard but does not release, while the outer barely touches. That is a classic sticking-caliper sign; see sticking caliper symptoms. The reverse (outer worn, inner fine) usually means seized slide pins keeping the caliper body from floating back.
One wheel worn far more than the other side
A big side-to-side difference points to a hydraulic problem - a collapsed flex hose or a stuck caliper on the worn side doing more than its share of braking. Fix the root cause before fitting new brake pads, because fresh pads on a bad caliper simply repeat the pattern. Reading the wear first saves you a second brake job.