Loudspeakers, like microphones, are transducers based on moving diaphrams and operate in the same manner except that the principles are reversed.  Whereas a microphone converts the movements of its diaphram due to sound energy into electrical signals, loudspeakers convert electrical energy into motion of the diaphram and thus into sound energy. 
Like microphones, loudspeakers use two different types of transduction:
the moving coil and condenser or electrostatic technique. 
The most common loudspeakers use a moving coil design where an electrical coil attached to a diaphram is suspended in a magnetic field. When a musical signal in the form of fluctuating electrical current is applied to the coil it moves in the magnetic field causing the diaphram to move back and forth displacing air and generating sound waves analogous to the electrical signal.  Electrostatic speakers use the same principle as condensor microphones only in reverse using the familiar fixed back plate and moveable diaphram plate across which are applied the audio signal voltage to induce movement in the diaphram 
Judging Loudspeaker Quality:
Frequency response. Ideally a loudspeaker's response should be flat over the range of human hearing. In reality, if the amplitude of any frequency reproduced by a speaker varies plus or minus 4 db from about 40 Hz to a5 kHz this would be considered acceptable and accurate tonal reproduction.  Dispersion. The angle of of uniform loudness in front of the speaker gives its coverage angle. This is the angle at which the output has dropped more than 6 db from its on-axis level, typically 60 to 90 degrees. Uncontrolled or wide dispersion is undesireable because in a studio or concert environment you want to hear what's coming off the tape or microphone and a minimum of reflections from surrounding surfaces which can unduly colour the sound.  Sensitivity or efficiency is the sound pressure level a speaker will produce at a distance of 1 meter when driven with 1 watt of pink noise. Typical values range from studio quality monitors with sensitivity of 93 db SPL to home bookshelf speakers with sensitivities of 85 db SPL.  High output capability describes a speaker's ability to reproduce high levels of sound without either distortion or damage to the speaker itself. It's sometimes necessary to listen back at high levels in order to hear the quiet details in recorded music so you want to have a speaker capable of approximately 110 db SPL output.  Transient response describes a speaker's ability to track the very fast changes in a sound's attack and decay. 
An important practical consideration when using any multi-speaker setup is that of speaker phase. If the same signal is applied to two speakers in phase with each other the cones will move in the same direction at the same time, therefore agreeing and reinforcing each other. On the other hand if the same signal is applied to two out-of-phase speakers one speaker cone will be moving outward while the other moves inward with a consequent tendency for the two speakers to cancel each other out. The sound of out of phase speakers will tend to be hard to locate in the stereo field and lack low frequency energy. The solution is to reverse the leads connected to one of the speakers while leaving the leads on the other speaker the way they were. Most speaker terminals are marked with + sign or a red mark to indicate polarity and speaker wire often has identifier markings on one strand to indicate its polarity. 
Loudspeaker impedance is an important consideration when considering connecting more than one speaker to one output of a power amplifier. The rule of thumb when adding speakers to an amplifier output is that, assuming the speakers have the same impedance ratings, the total impedance is the value of the individual speakers divided by the total number of speakers. For example, if four speakers rated at 16 ohms of impedance are wired to the same amplifier output the total impedance seen by the amplifier would be 16 divided by 4, or 4 ohms. 
Amplifiers have minimum output impedance values, usually 4 ohms for each output channel. They should not be operated lower than this value although they often (hopefully) have protective circuitry to prevent damage should a mistake be made. 
monitoring level
cables & connectors