There are actually quite a few different reasons why brakes may squeak. Here are some of the possible causes: 
1. Worn Out Brake Pads. Brake pad manufacturers add small tabs known as brake squealers to the front and rear brake pads. These tabs are designed to cause a squealing sound when the pads wear down to the point that they need to be replaced. This alerts the driver that it is time to get new brake pads, ensuring that they don't drive their car with worn-out pads.
2. Brand New Brake Pads. Sometimes new pads squeal when they are first installed. The squeak or squeal should go away after you drive the car for a while.
3. Long-Life Brake Pads. Long-life brake pads are much harder than traditional brake pads. This can cause them to squeak when they are used. This type of squeaking is considered to be normal.
4. Hard Braking. If you brake hard, it can glaze the brake pads, resulting in a squeaking noise.
5. Rust. Even a small amount of moisture can cause rust to form on your brake rotors. The rotors are made from unfinished metal, so it is normal for them to rust. You may find, however, that you experience a slight grinding or squeaking noise until the rust wears away. 
There are a couple of different ways to address brake squeaking. First, if the squeaking is caused by rust on the rotors or by hard braking, try driving 1/10th of a mile with your foot lightly on the brake pedal. This should wear away any surface rust or glazing, eliminating the brake noise. 
If you have new brake pads, your only option is to wait until they break in. After a short while of driving your vehicle, the squeaking should go away. With long-life pads, however, you may be stuck with the squeak since it is considered to be normal for them.
Finally, if your pads are worn out, you should take your vehicle in to have them replaced.
If you have any doubts about what is causing the squeaking, it is always best to play it safe and have them inspected by a mechanic.