Many computers use wifi to connect to the Internet. They use a combination of hardware and software to do this. Every laptop you buy today will have this hardware and software. Most desktop computers use a wired connection to a broadband modem and do not have wifi built-in. It's possible to buy extra hardware to use wifi on a desktop computer.
To use wifi:
You either connect to a public wifi site or use a modem to connect to broadband Internet from a Internet provider (like Telstra).