wireless - How to Connect/Disconnect to WiFi Via Terminal

I just have a quick question that I have been researching for a while now...
How do you connect/disconnect to a wifi network in ubuntu via terminal? I tried several commands that failed. I have been troubleshooting this for about 55 minutes now, and the question still continues to baffle me.
FYI, I am using wlan0

Thanks Ubuntu :)

P.S. If I can provide any additional info, don't hesitate to ask.