Motion

Speed, velocity and acceleration

Speed is the distance travelled per unit time: $\text{speed} = \dfrac{\text{distance}}{\text{time}}$, in $\text{m/s}$. It is a scalar.
Velocity is speed in a stated direction (a vector), also in $\text{m/s}$.
Acceleration is the change of velocity per unit time:

$$a = \frac{\Delta v}{t} = \frac{v - u}{t}$$

measured in $\text{m/s}^2$. A negative acceleration (deceleration) means slowing down.