What's the difference between data and Wi-Fi in a contract?

I'm planning to get an iPhone 4s, and it comes with 100MB data and 2BG of data.
What's the difference? Is it like if I use all my data I have Wi-Fi to fall back on, or is it that data is when you're not connected to Wi-Fi! I'm so confused! Also is 100MB enough?

Data is like all the stuff you do on the Internet like watch videos etc and Wi-Fi you can do anything when I don't feel like using data in school I switch to Wi-Fi or when I'm out I usually go with my cousin so he turns on his hotspot and at home I have Wi-Fi and 100MB being enough depends on how you use it

I'm not sure why you have 100MB of data and 2GB of data. Is the 100MB for upload?

Anyways…

If you use Wi-Fi, then surfing the web or sending emails, etc.won't count against your data plan.

If, however, you are not connected to a Wi-Fi network, then surfing the web or whatever will count against your data plan.

Even though I'm connected to a Wi-Fi network most of the time, my phone still uses about 500MB of data a month. Fortunately, I have the 2GB package so I don't go over. I also don't surf the web or update Facebook, etc. That often, so I'm not using that much data anyways.