From the "Activity Monitor" I noticed that even when the computer is idle, and with all browsers closed, it is still sending data out (several hundred bytes per second). I want to know why this is happening and where all the data is going.
It could just be bonjour or some other background network process or daemon communicating. You don't have to be explicitly browsing the web to be using network traffic
Bonjour - Is a set of networking protocols to allow different machines to see each other with zero configuration.
Daemon - Is just a process that runs unattended in the background.
Several hundred bytes is a tiny amount of traffic, it's nothing significant. If you would like to monitor which ports are open, use "netstat" at the terminal