A vector quantity is a quantity of something which possesses both magnitude and direction. Magnitude is simply the size or amount of the quantity. For example, the magnitude of the quantity eight miles per hour is eight. Direction is meant in the ordinary sense, where the quantity is headed.
An example of a vector quantity that should be very familiar is an expression like "go 10 miles east" when offering directions. In this example, 10 is the quantity and east is the direction. Both the direction and magnitude of the movement are necessary to get the person needing directions to the right location.
Vector quantities are used extensively throughout physics to describe physical systems and to calculate the effects that different parts of the system have upon one another. Examples of vector quantities used in physics include velocity, acceleration, force, lift and weight.
In physics, speed is considered a scalar quantity, a quantity that lacks direction and only has a magnitude. When a magnitude of speed is paired with a direction, it becomes a velocity. Direction is important. When two or more vectors come into contact with one another, their magnitudes are summed together depending upon their directions. Vectors which are moving in the same direction will have their sum magnitude increased, while those moving in opposition will have their sum magnitude decreased.