One has cellular the other doesn't.
The Cellular or 3G portion allows an iPad to connect to the internet using a Cellular network like the one a regular Cell phone uses. So you have internet anywhere you are without requiring a Wifi network to connect to.
Cellular iPads require a Data Plan from a Cell Carrier to use their Cellular portion. However their Wifi portion works just like al Wifi only iPad and does not require any additional plan. If you have Wifi at home it can connect to that at no additional cost to you.
You'll need to check with your cell carrier whether they have data plans for iPads. And if you are willing to pay for the extra service.