What is Data Allowance (Verizon)?

I have an iPhone with unlimited texting and the cheapest data plan.
What is data allowance? I keep on getting text alerts telling me that I've used 50%, 75%, 90% of the data in the pats two days.
What is it? What am I doing on my phone that is making this dumber go up? Is it Wi-Fi/ 3G usage?
Can someone tell me so I don't get charged for going over.

This is for 3g data only.

  • Will this add to my 3G data allowance? On my iPhone 4S on the settings tab where you turn on your 3G there's a button to turn Cellular Data on/off and another button for my 3G. If I only have cellular data on and not 3G will it add to my contract bill because all I have is 100MB so I really don't want to go over and pay more than I already have to so basically if I have cellular data on will it eat away at the 100MB I have?
  • Will any Verizon SIM card work in an Verizon iPhone 4S? I had recently acquired an iPhone 4S from an old friend. I was wondering if I could buy a cheap Verizon phone with a pay as you go plan, and remove the SIM from the cheap phone, cut it down to a Micro-SIM, and place it in my new iPhone.
  • When does data for a iphone from Verizon reset? And what uses up data? I just got an iPhone and I'm getting messages that say that the "usage reset date" is 10/10. Does that mean that is the day my data will be reset? Also, what uses up data? All I've been doing since I got my iPhone is texting and looking at a few things on the internet but I'm always connected to my Wi-Fi.
  • What is data in a Verizon data plan? I just got an iphone 5 and am having some trouble. I know that data includes email, accounts and any internet websites (facebook, youtube, netflix). Does it use up data to have multiple internet browsers open even though I only look at one at a time? Is it possible to use up data when the phone is off? Also, is taking a picture, playing app games, listening to music, and siri considered to be "data" on the data plan? Is there an overall explanation for what a data plan is?