My wife has a Samsung Tablet that has a SIM card installed. It talks to the cellular network and has a WiFi radio for talking to access points.
My Samsung tablet only has WiFi for talking to access points.
Imagine you working as a salesperson in a store:
Customer: "Can I use this tablet to access the internet when out of reach of a WiFi network"
SolarCabinChannel: "Yes, it has WiFi which means you can use it anywhere"
Customer: "ok, so why is this tablet over here $200.00 more and requires a monthly data plan?"
SolarCabinChannel: "I've been doing this for 50 years, I said it has WiFi didn't I, don't make me school you!"
Customer: "I'm just curious about the difference, I'm trying to learn"
SolarCabinChannel: "Well they both have WiFi, they each just have a different kind of WiFi. The more expensive tablet has more expensive WiFi than the cheaper tablet that only has the less expensive WiFi"