Many quantities measured in physics and physical chemistry can be classed as scalar or vector quantities.
A scalar quantity is completely defined by its magnitude and has no direction. Examples of scalar quantities are mass, volume, energy.
A vector quantity, however, does not merely have magnitude but also has direction. When a vector quantity is defined it must have both magnitude and direction. Straight lines can represent vector quantities both in direction and magnitude and examples are velocity, force, and momentum. Unless both are acting in the same straight line, vector quantities cannot be summed directly but can be combined to form a resultant vector by the use of the parallelogram of vectors. Two adjacent sides of a parallelogram can represent two vectors; the diagonal of the parallelogram drawn through the point at which the two sides meet represents the resultant single vector.