Every home has an electric panel, which is the control panel for all the circuits in the house. Power enters through two hot cables, each at a voltage of 120 volts to ground, which makes the voltage between them 240 volts. They connect to a grid in the panel to which each circuit is also connected. In modern homes, circuit breakers protect each circuit from overheating, but in older homes, fuses provided this protection.
The Fuse Box
Houses built in the early part of the 20th century had 120-volt service. Power came through the fuse box, which was usually a black metal box located in the basement or in a closet. The fuse box had a knife-blade disconnect, a 30-amp main fuse, and 10- or 15-amp fuses for the branch circuits. These fuses, which screwed into a ceramic housing, each featured a carefully calibrated metal strip that would melt when the current exceeded its rating. When a fuse "blew," the power would go off, and the fuse would have to be replaced.
The Breaker Box
Circuit breakers have been around since 1900, but because breakers and breaker boxes are expensive, and fuses work well, breakers didn't become popular until the mid-1950s. That's when the 60-amp service panel, featuring a 240-volt feed, was introduced. Breakers are reusable, which is a great advantage over fuses. Each one contains a magnetic circuit that pulls a lever to trip the breaker when the current exceeds the breaker's rating. It's a simple matter for a homeowner to reset a tripped breaker, but even though you can reset them, circuit breakers don't last forever.
Changing a Fuse
Besides the fact that the power went off, you can tell that a fuse has blown by looking in the small window in the front of its circular face. The metal strip, which is visible there, will be severed in half. To replace the fuse, you need one with same current rating -- don't be tempted to use one with a different rating because you don't have the proper one, or you could cause a fire. Unscrew the old fuse and screw in the new one. There's no danger, because the part of the fuse that you can touch is made of glass, which is an electrical insulator.
Resetting and Changing a Breaker
When a breaker trips, it displays a red flag, and to reset it, you need to turn it all the way off and then on again. Changing a circuit breaker is a job that should be preformed by an electrician, because it involves removing the panel cover. The inside of the panel is always electrically energized, even when the main breaker is off, and touching any of the wire connections can give you a serious shock. The electrician will pull off the old breaker and disconnect the wire, then connect the wire to a new breaker and snap it into the slot occupied by the old one.