A one-way mirror reflects some percentage of the light and lets some other percentage pass. It is a sheet of glass coated with a layer of metal only a few dozen atoms thick, allowing some of the light through the surface (from both sides). It is used between a dark room and a brightly lit room. Persons on the brightly lit side see their own reflection - it looks like a normal mirror. Persons on the dark side see through it - it looks like a transparent window. It may be used to observe criminal suspects or customers (to watch out for theft). The same type of mirror, when used in an optical instrument, is called a half-silvered mirror or beam splitter. Its purpose is to split a beam of light so that half passes straight through, while the other half is reflected — this is useful for interferometry.
Contrary to popular belief, one-way mirrors that function well between equally lit rooms do not exist. The laws of physics do not allow for real, passive one-way mirrors (ones that do not need external energy); if such a device were possible, one could break the second law of thermodynamics and make energy flow from a cold object to a hot one, by placing such a mirror between them. (There is no prohibition against one-way windows, however. Optical isolators are one-way devices, that are commonly used with lasers.)