What features comes with a 3G sim card?

So I was wondering what all does a 3G sim card do, because I was thinking about switching my sim card to a 3G one.
My dad went to our local teleshop and asked about the 3G sim but the sales person who helped my dad out said the 3G sim was only for the able to use the phone for VGA calling. I thought it was for the internet connection to be 3G also but the sales person only told him it was for VGA.

So I wanted to really know what a 3G sim card could really do and what is all the feature it can do.