What is the difference between a scalar and a vector?

A scalar is a value that has only a size (otherwise known as magnitude), not a direction. A scalar is unaffected by the direction in which the object concerned is travelling. Examples include kinetic energy, speed and distance. A vector on the otherhand, has both size and a direction. The direction in which the vector is facing affects the value of the vector, even taking a negative value if it's travelling in the opposite direction to what you take as positive! Examples include momentum, velocity and displacement.