Performing any of the following while connected to cellular data instead of wifi:
Watching movies not actually saved on you phone. Listening to music not actually saved on your phone. Receiving emails. Connecting to any social media networks such as Facebook, Snapchat, Twitter, Instagram, etc...
There are limitless ways for your phone to use data.
Anything the phone does except make a regular cell phone call or receive sms (green) messages. This includes things the phone does in the background when you are not even using or touching the phone.
That is a good question, cadreamer75. Some examples of data usage would be streaming a movie, checking email, surfing the web or social media sites, sending iMessages, etc. Does this help you understand how data can be used?