The word [flexion] comes from the Latin [flexere] meaning "to bend". In anatomy, flexion is the reduction in the angle between two bodily components that are communicated by a type of joint.

By contrast, [extension] refers to the opposite action, that is, the increase in the angle between two bodily components that are communicated by a type of joint.

The image shows flexion of the head, the upper extremity, and the lower extremity. Hover over the image to see extension of the same structures. 

Excessive flexion (hyperflexion) or extension (hyperextension) of a joint can lead to potential pathology as would be the case of hyperextension of the neck as a result of a car crash (whiplash injury)

Note that in a human in the anatomical position, flexion of the upper extremity is an anterior movement, while flexion of the lower extremity is a posterior movement. You could make a case that in these image the upper extremity is performing an anteflexion (anterior flexion) while the lower extremity is performing a retroflexion (posterior flexion). 

In the upper and lower extremities there are whole groups of muscles that, because of their action, are called flexors or extensors.