Displacement ($s$) is the distance between some object and its starting point, that has both distance and direction, and is usually measured in $ m $.

Speed ($v$) is the change of distance per unit of time (usually seconds), and is usually measured in $ ms^-1 $.

Velocity ($v$) is the change of displacement per unit of time, and is usually measured in $ ms^-1 $.

When an object is moving in a straight line (usually straight down), several equations can be applied to it depending on what is constant throughout the movement: the acceleration or the velocity.

Uniform velocity

$$v = \frac{\Delta s}{\Delta t}$$ $$a = \frac{\Delta v}{\Delta t}$$

Uniform acceleration

Note that the value of acceleration for gravity is $g$.

$$v = u + at$$ $$s = \frac{u + v}{2} \times t$$ $$s = ut + \frac{at^2}{2}$$ $$v^2 = u^2 + 2as$$

Terminal speed

Terminal speed is the speed at which both the forward force of an object and its backwards force are equal.