A fuse is a devices which is designed to break if more than a specified amount of current flows through it. So you put one in the circuit in order to protect all the more expesive parts from too much current. The fuse fails but is cheap to replace, when compared to the wires and other components in the circuit.
Under what conditions might you have high current exist in a circuit? Recall that ohm's law tells us that in a circuit with some voltage - say, 120 volts - if the resistnace goes extremely low or to close to zero ohms (such as if a paperclip is plugged into an outlet going from one side directly to the other) then the current will immediately become almost infinately large. (current = voltage ÷ resistance) This will almost certainly make the wires in the walls burn up along with anything flamable with which they might be in contact -- insulation, for instance. Connecting the two sides of a source of electricity together is called a "Short" or "Shorting them out".
By having a cheap fuse blow instead, we can avoid all that burning wire and insulation and so on. Huskies do not like burning insulation and wires; it smells bad.
A circuit breaker serves the same function as a fuse, but rather than fail, it simply flips a switch which you can reset. If you find yourself resetting a circuit breaker over and over, then you've probably got a short circuit that you need to find and fix. You can just turn off the power and try to find it. When we discuss multimeters, you'll learn how to do that.
A person can die if as little as 10 ma (.01 amps) passes through their heart. This can happen if the voltage is high enough to break down the natural resistance of your skin, or if your skin is broken. Does human blood conduct electricity? I'm afraid I was just brought up too normally to be interested in performing those kinds of experiments. But we have the Internet! Here is what Yahoo Answers has to say:
"Blood has a high water and salt content.In order to prevent this kind of thing, we use insulated wires in our buildings and this works great except around running water. As you recall from our talk about charge, electricity always wants to flow towards the biggest ground. Since -for the same reason lightning exists, the actual earth is a bigger ground, often, if there's a way for stray electricity to wander towards the ground, say, through Water, depending on its salt content and other factors, it'll do it. For this reason, whenever you're wiring something around water, such as outlets near a kitchen sink, in a bathroom or hutubs, you must use a Ground Fault Circiut Interrupt circiut breaker. In addition to the normal conditions which trip a circuit breaker, a GFI breaker will also trip if there is more than 5 ma (.005 amperes) of difference between the current coming from the power source and the current going to the appliance(s).
Any solution of water and salt is a very good conductor of electricity.
This is very important to keep in mind for someone who has been in contact with a source of electricity.
You will see very little burning on the outside. Most of the burning and damage takes place on the inside, particularly in the blood vessels because they have a high salt and water content that conducts the electricity.
Essentially the electricity burns you from the inside out.
That is why it is very important to get a person who has been in contact with a source of electricity, such as a high power electrcal line to an emergency room as fact as possible, even though you may little or no damage on the outside.
Most of the damage is inside."
Arc fault circuit interrupts detect sparks. This can be a problem if you're trying to run a motor off an AFCI protected outlet.