Motion is change in location over time. You can break down even fairly complex actions such as walking into a combination of many small points and masses changing locations at different rates and in different directions. Important concepts to introduce along with the concept of motion are force and conservation of linear momentum. Force is the cause of all change in motion: starting, stopping and changing speed or direction. Similarly, if no external force acts on a closed system of constant mass, the conservation of linear momentum dictates that its total velocity will not change.
Velocity, unlike speed, has both direction and quantity associated with it. Strictly speaking, adding external force to a system changes the system's total velocity, not necessarily its speed. A force may cause an object to end up moving at the same speed in a different direction. In baseball, for example, a batter could hit a 60 mph pitch so hard it flew at 60 mph toward the outfield. The speed would be the same before and after the hit, but the velocity would be different since the ball changed directions.
Acceleration is a kind of motion in which the velocity changes over time. Acceleration results when you apply force to the moving object or system. When introducing the concept of acceleration, it is useful to clarify that acceleration is not limited to speeding up. Negative acceleration, or deceleration, causes a reduction in speed. Reinforce the link between force and acceleration. Where velocity is changing, some force is acting. Where no force is acting, the velocity of a constant mass cannot change.
Complex motion occurs when a force at angles to the initial velocity causes a change in direction. If a billiard ball is rolling in a straight line, for example, another ball hitting it from the side will deflect it and cause it to go off in a different direction than it started from. When a weight is whirling on a string, the string keeps applying force toward the center, which makes the weight constantly change directions and whirl in a circle instead of a straight line.