Negetive Acceleration?

What does it happen when an object have a negetive acceleration? And is it possible to have a negetive acceleration?

A negative acceleration is just an acceleration in the opposite direction. Which could really be in any direction, since it is up to you to define forward, and acceleration is a vector so signs only mean a difference in direction.

Say a car was moving forwards, with no acceleration, just constant velocity, a negative acceleration would slow it down, because the velocity would be changing the same as the acceleration i.e. backwards, and then once the speed was 0, the car will speed up but backwards.

Not much strange about that.

Now, negative mass, thats weird.