How can I connect to a wireless network using only the command line in Linux?

The exact commands will depend on the network you're connecting to. I generally first use 'iwlist' to find a list of available networks.

iwlist wlan0 scanning

This will return a list of all wireless networks seen on interface wlan0.

The next step is to set the ESSID and what access point to use (usually automatic).

iwconfig wlan0 essid my_network
iwconfig wlan0 ap any

If the network is unsecured you're done configuring the wifi at this point. You can then set a static IP (ifconfig) or a request a dynamic IP (dhclient).

If the network is secured with WEP you'll need to enter the correct key.

iwconfig wlan0 enc s:my_network_key

If the network is secured with WPA(2) you'll need to install and configure wpa_supplicant first.

As always the manpage is the first place you should look for more information.