Nearly every driver is concerned with safety in their vehicle, and in terms of mechanical parts that can cause safety issues, the brakes are probably at the top of the list. Nobody ever wants to imagine the panic of pressing down on the brake and having it be unresponsive. This is why it's important that all drivers pay close attention to their brake system and know the signs that can indicate wear.
For starters, drivers should know that hearing a squeal from their brakes does not necessarily indicate a problem with the vehicle. Today's brake pads, especially those used in front-wheel drive vehicles, are semi-metallic, so it's natural to hear a bit of squeaking when the pedal is pressed down. In fact, many of the more expensive brake pads on the market will actually be a lot noisier than the cheaper pads you can buy. This is because they are made of a tougher material that does not wear as easily. If you can deal with the noise, it's a smart investment.
That being said, squeaking can indicate that your brake pads are beginning to go. It's probably something you should look into if the sound gets noticeably loud or unbearable. Even if there's nothing wrong with the brakes, the mechanic may be able to install a shim or suggest ways to reduce the noise.
Some noises, however, can indicate a serious problem. If your vehicle has disc brakes and you hear a clicking noise, then you'll want to take your car in for maintenance. This is usually related to the caliper, which holds the disc in place. Most of the time, the caliper only loosely fits over the disc brake, which is why springs are installed by the manufacturer to keep things in place and reduce the noise. If you start hearing a clicking, it's likely that one of these springs has become brittle and broken.
A grinding noise should be pretty obvious when you hear it, and this indicates that the brake pad has completely worn out. Usually, this is the next, more serious step after the squealing begins. It's a good idea to take your car in for brake service as soon as possible if you hear grinding, as driving it further could damage more expensive parts than simply the brake pads.
If you did let your car's brake pads go too long without maintenance, you may eventually start feeling a pulsation when you press the pedal. This indicates that the brakes are likely warped due to wear. Excessive heat in the brakes can also lead to warping over time. This can be an expensive replacement, so it's best to pay attention to other warning signs before letting things get this bad.
The winter can do a number on brake systems as well, and frozen brakes, calipers or brake cables are not all that uncommon, especially if you let your car sit outside when the temperature drops. While these can cause problems, they sound worse than they really are. A common symptom of a frozen brake is pulling slightly to the left or right when you go to stop. This indicates that one of the brakes is functioning correctly, but the other is lagging behind - likely because it's frozen. Other problems, such as brake fluid leakage or grease/oil on the brakes, can also cause this problem.
The emergency brake cables are a bit tougher to diagnose, but can become frozen too. Most people don't use their e-brake very often, but you'll be able to feel something's wrong if you suddenly notice a drag when you accelerate - as if an anchor was slowing your car down. Sometimes, this is simply because the person has not used their e-brake in quite a while - it's not a bad idea to pop it once or twice and see if that fixes the problem.