If the music has been imported into your iTunes Library, you can sync it to your iPod - it doesn't matter if you use a USB cable or Wi-Fi. The data goes from your iTunes Library to your device.
The technology is wireless LAN (local area network), the typical method of operating home and office Wi-Fi networks. You need to be on the same network because, as with any TCP/IP network, your computer connects to the router, your iPod/iPhone/iPad connects to the router, and the router connects the two devices.
You could also use
Home Sharing to play songs that are in your library (via Wi-Fi). Turn on Home Sharing in iTunes on the computer, sign into Home Sharing on your iOS device (Settings > Music, enter Apple ID and password) and as long as your iOS device and the computer are signed into the same Wi-Fi network, the shared library will appear in the Music app on your iOS device.
