Why is it that when I use wi-fi, it marks my usage under data usage?

[AC Question]

I just got my phone the other day (It's my first phone). I currently don't have any plan or contract or anything at all really. It's just the phone. I connected it to my home wi-fi the other day and it seems to have put my usage under data usage. Am I using data? Also, is this related to the icon just to the left of the battery percentage icon on the top of the screen?

 

[B. Diddy]

Welcome to Android Central! Are you talking about the Settings>Data Usage menu? That menu can show both mobile data (3G/4G) and wi-fi, so is it set to show wi-fi data usage?