WiFi is an abbreviation for Wireless Fidelity, which doesn't tell you much. It's just an easy way of saying "wireless networking".

Typically, computers are networked together using wires. If you look on the back of your PC at work, you'll see one wire running out the back of it into a plug on the wall. Looks like a phone wire, only slightly larger.

So, WiFi removes the wires. There's a few different types of WiFi: 802.11b, 802.11g and others. It basically just boils down to a network tho, allowing computers to talk to each other or the internet.