As a DOT3 or DOT4 brake fluid absorbs water, its boiling point decreases. It can absorb water from the air, which is why you should avoid opening your car's brake fluid reservoir. For the same reason, you should always keep containers of brake fluid tightly sealed.
DOT5 fluid does not absorb water. This means the boiling point will remain relatively stable, but it also means that any water that does get into your brake system will tend to form pure water pockets, which could cause brake corrosion.
Two other important things about brake fluid: DOT3 and DOT4 eat paint, so don't spill either of these on your car. Also, none of the different types of brake fluid should be mixed. They can react badly with each other and corrode your brake system.
For links to more braking articles, check out the next page.