What qualifies as Data on an Iphone 4s on a Verizon plan?

I just recently added a line to someone with an existing I-Phone plan, so now we both have the I-Phone 4S. We have unlimited texts and minutes, and have 6GB to share between the 2 of us each billing cycle. Apparently, she uses about 400MB of data a month. I don't know much about how data is measured or what it is, since I've never had a phone plan without unlimited data. I know videos uses a lot, but if you were to use, say Hulu or Netflix, when you are connected to Wi-Fi, would that still use up a lot of data? Some say using Wi-Fi means you aren't using data, it's only when surfing the web when using 3G, but I don't know for sure. Is 6GB a lot of data, and enough for 2 people? And is there anyway to track how much data each of us has used through out the month, to prevent overage?

Any transfer of data over the internet using the wireless network. Using Wi-Fi doesn't count against your plan.

Apps that use Data are anything that connects to the internet, some examples are:
-Movie Streaming (Netflix/Hulu)
-Music Streaming (Pandora/Slacker Radio/ iHeartRadio)
-Games (Words with Friends, any game that connects to allow other people to play simultaneously)
-Video Chatting (Skype/ooVoo/Fring)
-Social Media (Facebook, Twitter, Myspace)
-Using location based services (GPS)
-Checking E-mail

*When you're on a Wi-Fi Network you're not using the cellular data (network of your provider, Sprint, Verizon, AT&T) you're using your own internet so it's not counted on your 6GB of data*

The amount of data could be sufficient depending on how often you use your smartphone without a Wi-Fi network for your web surfing, video streaming, music streaming, video chatting (how often you use internet based tasks when you're not connected to Wi-Fi)

*To find out how much data you've used go onto your iPhone, click on Settings, go to General, then click on Usage and scroll down to the bottom it will have a data usage amount for both sent/received data over the cellular network

*If you're on AT&T you can have them send you a text message to tell you your data usage info by sending a text to *3282#

* You can go to myAT&Taccount.com and this will also show you how much Data you have used