Xbox 360 does not contain built-in WiFi networking support, you'll need to purchase an add-on external adapter if you want to network your Xbox wirelessly. The Xbox 360 wireless network adapter from Microsoft is one option. Microsoft's wireless adapter will be compatible with nearly any home network, as it supports all three of the popular WiFi local networking standards (802.11a, 802.11b, 802.11g).
Microsoft's Xbox 360 adapter supports the USB network interface, not Ethernet. USB network adapters in some situations are inferior to Ethernet ones, but not on the Xbox 360. For example, the USB interface supplies not only the network connection but also a power source, so no extra power cords are needed to make this wireless adapter work with the Xbox. Second, using a USB adapter keeps the one Ethernet port on the Xbox 360 free for another purpose.
In contrast, this adapter's support for 802.11a is less interesting. This is a high-end networking feature. A few hardcore home networkers may have the setup to take advantage of it, but most Xbox 360 owners will never need 802.11a capability.