That depends on the app. If I run Mail, it is going to check all my accounts routinely and use the Internet, if I run Textedit, it isn't going to use any since it's not Internet connected..
What EXACTLY are you trying to figure out?
Would be a lot more helpful to tell us WHAT apps you are running. You will want to keep an eye on your usage and determine which app is using the bandwidth by pruning your list one by one..there is no magic here..