Will I use data when on wifi?

I need help, I am confused.  Att says that you use the smartphone at a wifi location you wont be charged data usage, here are my questions

1. can it be any wifi or does it have to be ATT provided wifi, I have charter wifi 

2. Are they saying i can use my phone for posting facebook status, watching youtube, surfing the web, and it not use any of my data plan IF connect to wifi?

3.  Do smartphones find wifi (let says you are traveling)

One thing i want to use it for is like mapquest, If i am lost i want to be able to look up directions (lets say I am in a wifi area) will this work.  ALSO, if I am NOT in a wifi area i can still look up directions but it will us data usage right?

Please break this down for me, you wont offend me if you "spell" it all out for me

ALSO, what is the difference really between other smartphones and iphone... Just wondering, i am looking to purchase one.  I am disabled and thought this would be a good thing to have.  thanks