What's the Difference between these 2 Devices.?

Wi-fi is a network standard usually denoted by 802.xxx and it is a short range radio communication mechanism. 3G on the other hand is wide-range standard which uses almost the same communication mechanism (towers) etc as of GSM so it makes it possible to use high speed internet also on mobile devices like phone/laptops. GPRS is old and almost dying. 3G is the new standard and will be replaced by 4G and successors. Having a 3G device means you can use internet with more flexibility (on a journey) etc. While Wi-fi is short-range.