Friction is the force that opposes the relative motion or tendency of motion between two surfaces in contact. It acts parallel to the contact surface and opposes the motion or the tendency of motion. The correct answer is option 'D', which states that friction is parallel to the contact surface and opposes the relative motion.

Friction can be better understood by breaking it down into two main types: static friction and kinetic friction.

1. Static Friction:
Static friction is the force that prevents an object from moving when a force is applied to it. It acts parallel to the contact surface and in the direction opposite to the applied force. Static friction adjusts its magnitude to match the applied force until the force reaches a certain threshold called the maximum static friction. Once the applied force exceeds the maximum static friction, the object starts moving, and static friction is no longer applicable.

2. Kinetic Friction:
Kinetic friction is the force that opposes the motion of an object that is already in motion. It acts parallel to the contact surface and in the direction opposite to the relative motion. The magnitude of kinetic friction depends on the nature of the surfaces in contact, the normal force, and other factors. It is generally less than the maximum static friction.

Friction plays a crucial role in our daily lives. It helps us to walk, drive vehicles, hold objects, and perform various tasks. Without friction, it would be difficult to move or control objects. However, friction also has some disadvantages. It produces heat and wears down surfaces, leading to energy loss and decreased efficiency.

In conclusion, friction acts parallel to the contact surface and opposes the relative motion or tendency of motion between two surfaces. It is an important force that affects our daily activities and is vital for the functioning of various objects and systems.