Oscillators and multivibrators are electronic circuits that produce repeating signals. Though engineers and technicians use the terms interchangeably, strictly speaking, an oscillator produces sine waves and a multivibrator does not. Oscillator and multivibrator designs take different approaches to signal generation. The oscillator uses a resonant circuit and amplifier and the multivibrator is based on an electronic switch.
A bell resonates at a natural frequency. If you ring it, it produces a tone that fades away. An oscillator "rings" a special circuit that creates a signal at a specific frequency. An electronic amplifier sustains the signal, keeping it from dying out. The amplifier has its output connected to its input with the resonant circuit in the path. These circuits produce sine wave signals.
A multivibrator produces a signal with an electronic switch. When the circuit has low current, the switch turns on, sending charge to a capacitor. When the capacitor's charge reaches a certain level, the switch turns off and discharges the capacitor. The low current in the circuit turns the switch on again, repeating the cycle. This is an astable multivibrator, as it continually switches itself on and off. A bistable multivibrator remains either on or off after you switch it.
While some oscillators and multivibrators have a fixed frequency, sometimes having a general-purpose signal generator with a frequency you can tune for different applications is handy. Variable-frequency multivibrators are easy to design; a variable resistor in series with the capacitor slows down its charge rate by turning the resistor knob. A variable-frequency oscillator is harder to design. Its resonance circuits are finely adjusted, so changing the frequency can compromise the resonance if not done carefully.
Some oscillators incorporate a quartz crystal in the resonant circuit. The crystal has a resonant frequency of its own, and its natural stability allows for great precision. Radios, computers, clocks and watches use quartz-crystal stabilized oscillators as precise frequency sources. Multivibrators generally do not use quartz crystals.
An oscillator produces a sine wave as its primary output. Depending on the design, a multivibrator may generate sawtooth, triangle or square waves. By adding conversion circuits, however, you can produce any basic waveform from any input waveform. Given a triangle waveform, for example, three conversion circuits can simultaneously produce sine, sawtooth and square wave outputs. Having the ability to freely convert waveforms frees the designer to concentrate on other signal generation issues.