Fuses are used to cut of the current when it becomes too high, to protect the circuit. A fuse is usually just a piece of wire, that when the current reaches a certain point, the wire will melt, and the fuse will then need to be replaced. Fast acting fuses blow out from surges and extended high currents, while slo-blo do not blow out from sugrges, only extented high currents, and normal fuses are somewhere in between.
Some fuses are shown above. Most of the fuses have a way of seeing if the fuse has blown or not. In the first to the left, there is a window for looking at the wire. Other ones, such as those in the middle, are made of a glass tube that the wire runs right through. In the one on the far right, the wire can be between the two blades in the plastic.
NOTE: NEVER INSERT A FUSE INTO A DEVICE WITH A HIGHER RATING THAN THE DEVICE DESIGNED FOR. DOING SO CAN CAUSE DAMAGE TO THE DEVICE, AND WILL CAUSE AN INCREASED RISK OF A FIRE.