Velocity

One way of describing velocity, in a three-dimensional world, is a three-dimensional vector describing its direction and magnitude. This is because the velocity of an object is the speed of that object in a certain direction.
One can plot a graph of the velocity of an object by plotting its velocity in a given direction against the time. Usually, it will look similar to this.



As annotated on the graph:
Whenever a line in a velocity-time graph is perfectly straight (unlikely in the real world), the object is either:

• Moving at a constant speed, or
• Accelerating or decelerating constantly

Acceleration, put simply, is a change in velocity. An object's acceleration changes when either

• Its speed changes, and/or
• Its direction changes

Since both of these events change the object's velocity.
Here is an image, illustrating how to calculate the acceleration of a line in a speed/time graph