Stall (flight)

A stall is what happens when an aerofoil can not make enough lift to keep the aircraft in flight. Stalling can be risky and dangerous when caused unintentionally or with lack of enough vertical distance for recovery.
Cause.
A stall is caused when the critical angle of attack, or AoA, is exceeded. The angle of attack, which is the angle between the chord line and relative wind of the aircraft's wing, is typically around 15 degrees. 
Signs of the stall.
The signs of the developing stall are:
The signs of a full stall are:
There is the chance of entering a spin if the aircraft controls are not coordinated
Recovery from a stall.
To recover from a stall, the pilot must push the nose down. Then the pilot must increase the engine power using the throttle. When air speed increases again, the pilot can level the wings and pull up to return the aircraft to normal flight.
Deep stall.
An aircraft with a T tail is at risk from deep stall, because the turbulent air coming back off the stalled wings covers the elevators and makes it very difficult to push the nose down and recover.
Training.
In the United States the FAA requires private pilots to understand the theory and execute stalls at altitude. This, ideally, enables them to recognize the indicators of a stall and prevent an accidental stall at low altitude.
Power on stall (Departure Stall).
Used to simulate a stall while executing a take off, go around or climbing. 
The steps to execute a power on stall are:
Power off stall (Approach stall).
Used to simulate a stall while in a low speed "dirty" configuration(flaps out, gear down, etc.) such as while on downwind, base or final while prepping to land.
The steps to execute a power off stall are:
Accelerated Stall.
An accelerated stall is very similar to the power off stall, however it is performed while executing a turn. The term accelerated comes from the fact that while in a turn there is an extra "load" due to the G-force being exerted on the plane. This extra force makes it easier for a stall condition to occur.
Stall turn.
A stall turn is an aerobatic manoeuvre also known as a hammerhead. The pilot lowers the aircraft's nose to increase speed, then pulls up sharply to the vertical. The aircraft slows to a low airspeed, then the pilot applies rudder to yaw to the left or right through 180 degrees. The pilot then recovers from the subsequent dive once the aircraft has sufficient airspeed. If executed correctly the wing is not stalled at any point, hence the preferred name of "hammerhead" for this manoeuvre.