Inside a typical restaurant pager is a very simple circuit board with just a few basic components:
A simple radio antenna, made from a coil of wire wrapped around a metal core, picks up the signal from the master transmitter. This signal is sent to the microprocessor, where it is compared against the CAP code for that pager. When the signal matches the CAP code, the pager alerts the user using one or more of three methods: audio, visual or vibratory.
An audio alert usually plays a tone or series of tones through a tiny piezoelectric speaker mounted directly on the circuit board of the pager. Some pagers actually play a prerecorded voice alert, such as "Your table is ready." In many pagers, a series of LEDs flash rapidly or simply light up when an alert is sent.
As for the pagers that vibrate when activated, inside is a small DC motor:
You can see that a small weight is attached to the motor. This weight is mounted off-center on the motor's spindle. When the motor spins the weight (at approximately 100 to 150 rpm), the off-center mounting causes a strong vibration.
There you have it. The next time you have to wait for a table at a restaurant and the hostess hands you a pager, you will have a whole new appreciation for the elegant yet simple technology that lets you wander while you wait.