If you want to know more about the WiFi connection you are currently attached to, you can hold the option key while selecting the WiFi icon in the menu bar. You get information like the mode, (802.11g, 802.11n, etc), channel, security, transmit rate and your ID on the network.
Related: Detailed Wi-Fi Information, WiFi Indicator Extras, Lion : iMac does not automatically connect to my WIFI network !, Using Airport Express to provide Ethernet connection from existing WiFi network?, Establishing invisible wifi networks.
