A spin is an autorotation of the aircraft, normally during descent. The aircraft must stall
before it can enter a spin, and one wing is stalled while the other is not, causing the aircraft to rotate around the vertical axis
while descending. Note that the airspeed will not read very high, though it's likely inaccurate.
A normal spin is with the nose down, and in aircraft approved
for spins is generally fairly easy to recover from, once the pilot gets past the disorientation that almost everyone experiences their first few times in a spin. In other aircraft it may be more difficult or even impossible to recover from a spin.
A flat spin is with the nose more or less level (or even a little nose up), and is difficult to recover from in most aircraft, and is impossible in many aircraft.