iPad with Cellular means the iPad has a SIM slot like a phone and can connect to the cellular data network, like your phone. A WiFi Only iPad can only connect to data (and say, browse the internet or download emails) if there is a WiFi network available which you can connect to. So if you plan to be out an about a lot with your iPad, getting one with cellular can be a great option.
Now, we have to be careful here because unlike a phone, iPad with Cellular only connects to data and not to voice or standard text messages (text messages use the voice network channel on your phone). If you use iMessage, you can send and receive text messages to other people using iMessage; these look like text messages but are actually sent over mobile data.
The upshot is that even with a functioning SIM card your phone won't take normal phone calls like a cellphone. That said, these days there are a ton of apps like Skype, FaceTime & WhatsApp to name a few, which enable you to make calls over the data network, and more and more of your contacts will be using these apps too.
The other advantage of iPad with Cellular is it comes with a GPS antenna, unlike WiFi Only. This means you can use your iPad with Cellular for maps and navigation which is great if you're going to use it on a boat or plane, or just for walking around touring a city - just make sure your get a water proof case if you're taking it out on the seas!
*** Pro Tip ***
You can quickly tell if an iPad has cellular by looking at the top back of the iPad - if there's a thick black or white band like the one below you know it has cellular (that's where all the antennae go...)