When the iPad first came out it had an unlimited data plan option with AT&T. If you activated that plan and never stopped it, you could have unlimited data.
AT&T discontinued that plan within a few months, and now only offers a 250MB plan for $14.99 and a 2GB plan for $25.00
To see what plan you have, go into Settings, choose Celluar Data, and turn it on. Now choose the View Account button. You'll need to sign in with your Apple ID (account email and password). Or maybe I just used the same ones and forgot.
You'll be able to see what your current (if any) data plan is and how much data you've used, and what your billing period is. You can also buy or change a plan from here.
If you have unlimited and you change to another plan, you will not be able to go back to unlimited. If you don't have unlimited, then you are stuck with the two plans available now, or nothing.