Brake pads serve the purpose of a slowing a bike down. A common wear item on any bike, there are many different types of brake pads and it's crucial that you find a suitable replacement for those that came with your bike originally. Once you have selected the correct fitment, then there are options to consider which will effect braking strength, finite control, noise reduction and/or durability.
Disc brakes consist of a disc rotor attached to the wheel's hub, and a brake caliper fixed to the frame or fork. This type of brake is common in mountain, urban and now even road bikes. There are dozens of brake pad shapes and sizes on the market today and so the most important aspect of choosing a replacement disc brake pad is to ensure it's an exact match with your brake caliper. Knowing the brand and model name of your brake pad should suffice, but if in doubt, you'll need to remove the old pad and ensure the shape and size are the same.
Disc brake pads are typically available in two material compounds – sintered metal or organic/resin. The sintered metal is typically considered the more durable option and offers greater braking power. The resin/organic options are softer and so will wear faster, however, they also typically are quieter and offer greater finite braking control.
Rim Brake pads are made from rigid and long-lasting rubber and are designed to contact the bicycle's rim for slowing. Like disc brake pads, it's important that you choose the right fitment for rim brake pads too. While many options exist, the two key types on the market are 'road brake caliper' and 'mountain bike v-brake' pads. Pictured below is a mountain bike 'v-brake' style pad.
Additionally, more expensive bikes are likely to use pad 'inserts' where the rubber aspect of the pad is replaceable separately to the pad holder. If your brakes have a clear pin or bolt holding just the rubber pad in place, then it's likely you can replace just the inserts.