A stall must occur before a spin can take place.  The principle difference that transforms a straight-ahead stall into a spin entry is that one wing stalls more than the other.  This usually happens when there is an element of yaw as the stall breaks, and occurs most often when the airplane is in uncoordinated flight.  Stalling with crossed controls is a major cause of spins, and usually happens when either too much or not enough rudder is used for existing yawing forces.  If an airplane is near the stalling angle of attack, and if more lift is lost from one wing than from the other, that wing will drop more quickly than the other, creating a roll toward the wig with less lift.  As this wing drops, its local relative wind will come more from below, further increasing the angle of attack for that wing.  Likewise, as the airplane rolls around its center of gravity, the upper wing has a lower local angle of attack, and continues to develop some lift.  This situation of unbalanced lift tends to increase as the airplane yaws toward the low wing, accelerating the higher outside wing while slowing the inner, lower wing still more. As with other stalls, the nose drops, and and as inertial forces come into play, the spin usually stabilizes at a steady rate of rotation and descent.


Incipient Spin:  This is the first phase and exists from the time the airplane stalls and rotation starts until the spin is fully developed.

Fully Developed Spin:  Exists from the time the angular rotation rates, airspeed, and vertical descent rate are stabilized from one turn to the next.

Spin Recovery:  When the anti-spin forces overcomes the pro-spin forces.
It is crucial that you initiate recovery from an inadvertent spin as soon as possible, since many airplanes will not recover from a fully developed spin, and others continue for several turns before recovery control inputs become effective.  The recovery from an incipient spin normally requires less altitude, and time, than the recovery from a fully developed spin.  Keep in mind that although some characteristics of a spin are predictable, every airplane spins differently, and an individual airplane's spin characteristics vary depending on configuration, loading, and other factors.

Where the primary goal in recovery from a straight-ahead stall is to reduce the angle of attack to restore airflow over the wings, spin recoveries have the additional goal of stopping the rotation.  The complex aerodynamics of spins may dictate vastly different recovery procedures for different airplanes, so no universal spin recovery procedure can exist for al airplanes.  The recommended recovery procedure for some airplane is simply to reduce power to idle and let go of the controls, and others are so reistant to  spins that they can be considered spin-proof.  You should be intimately familiar with the procedures in the POH for the aircraft you will be flying.  Here is a general recovery procedure, but it should not be applied arbitrarily without regard for the manufacturer's recommendations: