Yes it runs in the background and starts upon login. There is a log store somewhere, not exactly sure where, a search in spotlight should bring up results though. You can open up the app window when you need to view the results so far
You input the start date of your month and maximum traffic limit and it calculates your usage from those
Its an excellent little app, though it wasn't for me, I'm looking for one that allows adjustments for daily usage as that is how it is tallied on our college network.
b.