All the above is true. Operationally, a stall happens when you slow
the airplane down and increase the angle of attack over
the wing so it is no longer producing lift. Student pilots practice doing them so they know how to get out.
It is not particularly dangerous to do stalls if you are careful about it, but I find them to be rather uncomfortable. There is a possibility of going into a spin if the pilot doesn't pay attention. But that, too, can be recovered from.
The other day I did a power-off stall. I slowed
the airplane down, applied full flaps, then reduced power. I raised the nose untill it
buffeted, then
the wing lost lift and the nose dipped. I lowered the nose some more and applied full power and flew away.
