The tape itself consists of a thin plastic base material, and bonded to this base is a coating of ferric oxide powder. This oxide is a ferromagnetic material, meaning that if you expose it to a magnetic field it is permanently magnetized by the field. Inside a tape recorder there is an electromagnet that applies a magnetic flux to the oxide on the tape. The oxide permanently "remembers" the flux it sees. A tape recorder's record head is a very small, circular electromagnet (consisting of an iron core wrapped with wire) with a small gap in it. During recording, the audio signal is sent through the coil of wire to create a magnetic field in the core. At the gap, magnetic flux forms a fringe pattern to bridge the gap and this flux is what magnetizes the oxide on the tape. During playback, the motion of the tape pulls a varying magnetic field across the gap. This creates a varying magnetic field in the core and therefore a signal in the coil. This signal is amplified to drive the speakers. n a normal cassette player, there are actually two of these small electromagnets that together are about as wide as one half of the tape's width. The two heads record the two channels of a stereo program. More info here: http://electronics.howstuffworks.com/cassette.htm