Microphones are devices that convert the mechanical energy of a sound wave into an electrical signal that can be amplified and sent to a speaker to reproduce the origianl sound, only louder.
Several technologies allow microphones to convert mechanical energy into electrical signals. One common microphone technology depends on magnets; this type of microphone is called a dynamic mic. Another microphone technology depends on capacitors; this type of microphone is called a condenser mic. In either type of microphone, a thin membrane or diaphram (with a surface area that's large with respect to its mass) moves when a sound wave strikes it (remember that sound waves are compressed air).
A dynamic mic, creates an electrical signal when the motion of the diaphram causes a magnet to move past a coil of wire, inducing a small electrical signal in the coil. The electrical signal mimics the motion of the diaphram and, therefore, the original sound. Amplified, the signal can drive a speaker that recreates the sound.
A condenser mic creates a similar electrical signal, only the diaphram interacts with a capacitor (which is made up of two electrically conducting surfaces separated by an insulator that will let current through if the charge on the conductors gets large enough) instead of a magnet and coil of wire. One plate of the capacitor in the microphone receives a small electrical charge from the PA system to which its attached (or a battery in the mic). The motion of the diaphram allows the charge on the first plate of the capacitor to overcome the insulator and reach the second plate. The pulses from the transfer of charge mimic the motion of the diaphram and, therefore the original sound. Amplified, the signal can drive a speaker that recreates the sound.
Unidirectional mics are microphones that, more or less, pick up sound waves from only one direction. Omnidirectional mics are microphones that, more or less, pick up sound from all directions. No microphone is truely unidirectionl or omnidirectional. However, the concept of directionality helps in the selection of microphones for particular applications. The directionality of the mic is independent of whether it is a dynamic or condenser mic.