What is Wi-Fi and its application?

Hi elec350, wifi is wireless fidelity. Most probably you have heard about WLAN. It is just the same.

Why don't you take a look at www.wikipedia.org and search for WiFi. You will get the overview you need.

Wi-Fi does NOT stand for Wireless Fidelity. It doesn't stand for anything. It certifies that different wireless products based on the 802.11 standard work properly together. For example, if you have a Wi-Fi certified laptop you should be able to use any other Wi-Fi certified network around the world. It doesn't mean the WLAN itself but a certification that a specific WLAN is Wi-Fi certified. A product is Wi-Fi if it fulfills certain requirements specified by the Wi-Fi Alliance. But people often use Wi-Fi to designate WLAN.

What Jone told is quite right. However, in loose (:D) sense (not official) Wi-Fi used as Wireless Fidelity.