Hi and welcome to the forums!
Background data does count towards your data total. For example, if you pin your music from Google Music to your device, it will download them in the background; same for news apps that auto-update every x hours. Even though you aren't necessarily using the data right then, your phone is still pulling it.
If you are concerned with data usage, make sure that you have WiFi enabled and connected and uncheck auto sync features in apps if not needed.