Why does the Usage Meter show usage even when I am not actively using my Internet service?

Your computer may be sending or receiving data even if you are not actively using your Internet service.  For example, your computer could be automatically downloading operating system updates, security software updates, and application updates like iTunes, games, etc.  Also, if you use a cloud-based storage service and have configured that service for automatic uploads to back up files, this will also add to your data usage.