Does turning cellular data to ON on the Iphone 5 use data, even on Wi-Fi?

I have a very limited data plan (only 2g a month). I understand using Wi-Fi limits it. What exactly does cellular data do. My understanding is it just uses data when Wi-Fi isn't applicable. Is this correct? And does having it on use a lot of data, eben if i'm on Wi-Fi?

If both data and Wi-Fi are on, your phone will use Wi-Fi provided there's a known hotspot near by and it shouldn't use any data. However, I would turn the data off anyways just to make sure, and it will save battery if its off

Having data on enables your phone to connect to a data network if necessary (3G, Edge, etc.) having it off will not use any of your data plan. Data is typically not used if you're connected to a Wi-Fi network.

Also, your data plan is actually probably 2GB, or 2048mb, which is actually quite a lot. More than I could use in a month if I tried.