What is part of data usage for iphone?

I have an iphone and i got a notification that i've used up 90% of my data.
What activities/things that i do on my iphone are considered data? What things on my iphone uses up data?

Roaming, Internet use without being connected to Wi-Fi (3G/LTE), texting

There's a switch in your settings that will turn off your "Cellular Data", turn that off and see what stops working.
But basically anytime you do not have Wi-Fi access your cell data includes:
any browsing of internet, opening webpages, streaming videos, downloading emails, social apps (like facebook), pretty much anything that connects you to the internets.

Roaming, email, internet connections, receiving app data, downloading/uploading data, etc, etc.

It is possible to turn off data through settings.

The bottom line is most text is pretty meaningless (messages, emails, etc.). Use apps instead of browser for reading news, sprots, facebook, etc and the biggest use is streaming stuff like video and audio, use Wi-Fi for that.

I'm using a new iPhone. I have put data on it twice, and both times the data disappeared into thin air; two days ago I put on air time which I never used; also disappeared into thin air. I never use it outside of Wi-Fi. What can be my problem?

Just been to my service provider; apparently the background updates are the reason for losing my data. Time will tell.