Correct me if i'm wrong. Buzzers can only produce 1 tone but are quite loud. While piezos can generate a wide variety but are most suited to ics. Is there anything i missed?
By piezo you most likely mean a piezo speaker, is a electromechanical device that converts electric energy (AC voltage) into sound energy (sound wave) and vice versa. Buzzer name is used as a device that needs a DC power supply that has in built a tone generator, power driver and piezo speaker. This tone generator and power driver can be one transitor, capacitor and some resistor.
In general, if you're talking about modern audio signalling devices, buzzers must be externally driven, while beepers contain an internal driver and merely need a DC power source to operate.
Some buzzers comprise a miniature loudspeaker-like device to generate sound and can be driven over a range of frequencies. However, since the purpose of the device is to get attention, the housing and transducer are usually tuned for the purpose of generating a single loudness-optimized tone at a specific frequency within the range of human hearing.
A piezo buzzer, on the other hand, comprises a piezoelectric ceramic material bonded to a thin metal diaphragm. When an alternating electrical signal is impressed across the ceramic its dimensions change and force the diaphragm to deflect back and forth normal to its surface. This transducer's resonant frequency is determined by the material characteristics of the coupled ceramic and metal elements and is also usually housed in an enclosure tuned to the resonant frequency of the transducer.
The third wire is used to generate feedback at the resonant frequency of the transducer, making it easy to use the transducer in a "self-driven" oscillator.
Beepers comprise a family of sounders with their drivers integrated into the buzzer package, and merely need to be connected to power and perhaps to an "enable" signal to produce an audio output.
A notable exception is the original buzzer, which was essentially a form "B" DC relay with the contacts connected between the supply and one end of the coil, with the other end of the coil being connected to the other side of the supply when a buzz was wanted.
These were widely used as electrical continuity testers until the advent of semiconductors, since the high-voltage transient caused by the opening contacts could, and can, easily destroy diodes and transistors.