Wi fi is like the connection you have in your laptop, so you can only connect to the interent when you are near a wi fi router, such as in your home, or in a building like a hotel or pub that offers wi fi.
3G is like the connection you have in your "internet enabled" mobile phone, so if you have 3G in an ipad it is like walking round with a 3G mobile phone so you can connect to the internet anywhere, but it does cost.
Note you have to pay a monthly cost for the 3G service from a mobile provider like you would in a mobile phone, so it is not JUST the extra cost of 3G in the ipad but a monthly ongoing cost.