This is a glass ferrule type fuse. It fits great with our fuse holders. The fuse is rated for 250VAC. Fuses are pretty simple, if more than a certain amount of current passes through the fuse, the small filament inside the fuse heats up and breaks (like a light bulb burning out). The circuit is then broken and hopefully your circuit is saved from molten destruction.
For smaller loads, you may want to checkout a PTC (aka resettable fuse). They handle less current (500mA) but can be set and reset many times without doing harm to your system.