According to the Tate glossary the word kinetic means relating to motion. Kinetic art is art that depends on motion either from wind, light, water or mechanical manipulation for its effects.
Artists have incorporated motion in art since ancient Egyptian times. In modern times mixing art with movement was explored by such artist as Alexander Calder, Marcel Duchamp and Naum Gabo. During a period where technology and machinery gained popularity so to did kinetic art.
Kinetic art considers the way that art looks when combined with motion. Sculptures are the most typical expression of this art form. Three dimensional works which produce moving shadows or are actually in motion themselves are characteristic of the spirit of this form. Op Art is often compared to kinetic art and many artist produce works that are interchangeably defined in both categories.
Two of my favorite contemporary kinetic artist are Tim Fort and Yaacov Agam.