The discovery of GMR opened new windows in the field of nano-materials, and nowadays research in these materials has become fashionable due to its tremendous technological potential and the deep fundamental physics involved in it. The discovery of giant magneto-resistance (GMR) has been a huge impact on our life, especially for mass data storage devices. Initial experiments conducted by Grunberg and Fert are explained. Basic physics of the GMR effect can be explained by the two-current model, which the conduction of a current is, consist of two different spin electrons. Details of GMR applications, such as hard-disk read-heads, sensors and magnetic memory chips are presented. One of the important aspects of GMR discovery was that it was immediately turned into commercially available products (the first GMR Hard disk head was introduced by IBM in 1997) with a giant market share. The highest storage capacity in the modern computers is attributed to the discovery of GMR. The story of GMR effect tells us how an unexpected scientific discovery could give rise to completely new technologies, new science and new commercial products, that means the new technology promises smaller and faster tools.