JEE

The scalar quantities are physical quantities with no direction and only magnitude (size). Examples of scalar quantities are mass, temperature, speed and distance. On the other hand, vector quantities have both magnitude and direction. The examples are force, acceleration, displacement and velocity.
The scalar quantities can be described by a single number and vectors require both a directional component and a numerical value. In understanding motion, vectors are important because they indicate both the movement's amount and direction. In systems which are more complex with multiple forces or directions, vectors help accurately to represent and analyze motion graphically and mathematically.

Distance and displacement both refer to how far an object has moved, but the meanings are different. The distance is a scalar quantity and it is the total length covered by an object, irrespective of the direction. It is always positive, and talks about how much ground the object has covered. On the other hand, displacement is the vector quantity and refers to the change in position of an object from the initial to the final point. It considers both the magnitude and direction. It can be negative, positive or zero depending on the motion's direction. If one starts from an initial point and covers a distance of 5 meters and then comes back 5 meters again, the distance covered is 10 meters and the displacement is 0 meters. 

Motion in a straight line is used to describe an object's movement along a straight line in a single direction. It is also known as the rectilinear motion. In Motion in a Straight Line, the position of the object changes along one dimension and unless an external force is applied, its direction stays constant. The important concepts related to straight-line motion are velocity, displacement and acceleration. Acceleration measures any change in velocity, displacement refers to the change in position and the velocity indicates the speed and direction. Examples include an apple falling straight downward, and a car moving on a straight road.