What is the difference between unlimited data and unlimited internet?

So i'm hopefully getting a new phone (iPhone 4s) for christmas. I have been searching over the past few months at the best deals. I use the internet e.g facebook, twitter, google, youtube quite alot so i needed a internet deal. I have found a deal that has unlimited texts and unlimited data, underneath it is the same deal but unlimited internet for a difference price. I thought data was internet?
Also, if you already have a Wi-Fi pass for bt or sky that you use on ipod touch, does the iphone work the same way? As in if you have internet connection (the password) can you use it? Like if i was in my house or would they charge me for it? Also is 3g free internet?

Nothing

There's none

Just used different wording means exactly the same

Should be the same however

unlimited internet - often means unlimited web pages, which work well with slow data
unlimited data - means fast and large amounts of data.

So you might have say 1GB data allowance (and can watch youtube and TV etc)
but once it has gone, you get internet only (slow speeds but fine for reading web pages, pictures can be a little slower to load but still fast)

T-mobile has unlimited internet for example.

They mean the same thing.