Automotive fuses don't "blow" all that often, but when they do, the results are often dramatic. Whatever happens to be on the same electrical circuit immediately stops working.
When a radio, headlights, cigarette lighter or any other electrical accessory suddenly stops working, a blown fuse is an immediate suspect. Of course, the cause may be something else, but the fuse block is a logical place to check first. What makes a fuse blow? Vibration or a temporary current surge or maybe a manufacturing defect.
If a fuse blows, replace it with a new one of the same physical size and amperage rating. If the new fuse blows immediately, then you've probably got an electrical problem somewhere. Have a mechanic check it out.
Where is the fuse block? Often it's under the instrument panel above the brake pedal. On some cars you may remove a little door that clips or slides in place. On others the fuse block is in the engine compartment. More often than not, you have to be a gymnast to reach it - to lie on the floor with part of your body outside the car. And you may need a flashlight.
If you have an owner's manual for your car, then you're in good shape. It should not only tell you the location of the fuse block, but the size and type of fuses to use. Even if you don't have an owner's manual, once you locate the fuse block you're okay. Beside or below or above should be some markings to indicate the electrical rating of each fuse and what it operates.
For example, "dome and cigar 15" next to a fuse would mean it's in the circuit that operates the dome light and the cigarette lighter, and its electrical rating is 15 amps. So you would want to replace it with another fuse with an electrical rating of 15 amps (and the same size and shape).
The fuse block, by the way, is simply a plastic block with metal clips that the fuses fit snugly into.
Usually you can tell if a fuse is bad by looking at the thin wire filament. If the filament is burned in two, you know it's bad. When in doubt about a fuse, replace it with a new one. If that doesn't solve the problem, then look elsewhere.
There are different ways of removing a fuse from the fuse block. A way that almost never works is grasping the fuse with your fingers, because fuses fit so snugly in their clips and are so close together you can't get a grip on them. You can bend a paper clip or wire into a hook and use it to pull the fuse out, or wrap a bag tie or bit of string around the fuse and pull it out, or even pry it out with a screwdriver. Take care not to break the fuse or touch the metal clips with the screwdriver, that can create a short-circuit, blowing out other fuses.
When buying replacement fuses (you should already have a few on hand, don't wait till a fuse blows), simply be sure they match. Numbers on the ends of the fuses refer to the electrical rating, letters to the length.
To put a fuse back in the fuse block, simply push both metal ends of the fuse into the clips with your fingers on each end of the fuse.