The reference documents for this Debian configuration file are:

• man 5 interfaces
• man wireless
• man resolvconf for resolvconf options (dns-nameservers, etc.)
• /usr/share/doc/wireless-tools/README.Debian
• /usr/share/doc/wpasupplicant/README.Debian.gz for wpa- options

See this two pages: wpa_supplicant in Debian 6.0 and How To Use wpasupplicant.

Simple configuration with automatic IP address and WPA security (credential can be in the same file, or in wpa_supplicant.conf:

allow-hotplug wlan0
iface wlan0 inet manual
    #wpa-ssid MyESSID
    #wpa-psk MyWPASecret
    wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf

Static IP address with WPA security; with this configuration the WiFi connection will be not managed by NetworkManager:

auto wlan0
iface wlan0 inet static
        address 10.0.0.166
        netmask 255.255.255.0
        gateway 10.0.0.189
        dns-nameservers 62.48.51.6 151.99.125.2
        #wpa-ssid MyESSID
        #wpa-psk MyWPASecret
        wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf

auto eth1
iface eth1 inet static
        address 192.168.0.191
        netmask 255.255.255.0
        gateway 192.168.0.249
        dns-nameservers 62.48.51.6 8.8.8.8
        wireless-essid MyESSID
        wireless-mode Managed
        wireless-key off