If you observe a moving bus, you will notice that the position of bus is changing with time. This means that the bus is in motion. Now suppose you are sitting in a bus moving parallel to another bus moving in the same direction with same speed. You will observe that the position of the other bus with respect to your bus is not changing with time. In this case the other bus seems to be at rest with respect to your bus.
However, both the buses are moving with respect to surroundings. Thus, an object in motion can be at rest with respect to one observer whereas for another observer, the same object may be in motion. Thus, the motion is relative.
Some objects move in straight line and some are not. For example, a ball rolls on a horizontal surface, a stone falling from a building, and a runner on 100 m race track. In all these examples, the position of moving objects is changing with respect to time along a straight line. This type of motion is called motion in a straight line or rectilinear motion.
In the motion of time hands of a clock, motion of child sitting on a merry-go-round, motion of the blades of an electric fan, an object follows a circular path during motion. This type of motion is called circular motion.
In the motion of the branches of a tree, they move to and fro from their central positions (position of rest). Such type of motion is called oscillatory motion. In such a motion, an object oscillates about a point often called position of rest or equilibrium position. The motion of swing and pendulum of wall clock are also oscillating motions.