The Wi-Fi seal of approval assures the end customer of interoperability with other network cards and access points which also bear the Wi-Fi logo.
To get Wi-Fi Certified the vendor has to join the WirelessEthernetCompatibilityAlliance (WECA) and submit their products to Agilent Technologies Silicon Valley Networking Lab (SVNL) so they can verify that the product meets the interoperability requirements. Agilent then notifies WECA so they can grant the certification.
You can view a list of Wi-Fi Certified products at http://www.wi-fi.org/certified_products.asp