Both of the above will use your cellular data, unless you are on wifi.
What phone and OS do you have? With Android, you can see exactly which apps are using the data by going to settings, and data usage. With iOS, it's going to give you categories, and that can help narrow down what is using the data.
You can also check the settings within apps to ensure that some things (such as downloading or playing video, backing up your photos and vides, etc) ONLY happen over wifi.
Like SuzyQ said check and see which apps are using up your data. I have a android phone and I have my mobile data off and some of the apps that I have set for wifi only will still use some data. I think this are apps that must get a signal from Verizon for the phone to work right. They don't use a lot of the data in a months time but they still use it even when I have the data turned off. The one app that I have found to use a lot is the play store. You can go into settings once you open the app and have it update your apps over wifi only. This really helps because with all your apps they could update at anytime an update comes out for a certain app. Another app is facebook but that has setting that you can put it to only download over wifi. I am sure others will come on here and give you other suggestion to keep you data to a minimum.