Is Data Slowed Down After Data Limit for AT&T with iPhone/Smartphone?

I'm looking into getting an iPhone with AT&T on contract.

I'm probably choosing the 1GB of DATA option.

What I need to know is: Is Data just stopped if I get to 1GB? Or is it just slowed down after that?
Is there any fee I have to pay after 1GB is reached?

Internet data doesn't slow down. Once you reach your data internet plan in this case 1GB, anything over there's a fee.