What is the difference between a scalar and a vector?

A quantity that has magnitude (size), but no direction, is a scalar. A quantity that has magnitude (size) AND direction is a vector.

Scalar quantities just have a magnitude. The direction does not matter.

Examples of scalar quantities are speed, distance, mass, and energy.

Vector quantities have a direction as well as a magnitude. They are often drawn as arrows on a diagram. When answering a question you may need to describe the direction (i.e a force of 3N straight downwards, a velocity of 7m/s North, a momentum of 16 kgm/s to the right). You may also need to resolve the vectors into their components before doing calculations with them.

Examples of vector quantities are force, momentum, velocity, displacement, and acceleration.

If an object is traveling at a constant speed, but changes direction (such as a car going around a roundabout, or a satellite in orbit), then its scalar speed will remain the same, but its vector velocity will change.