Arq has 2 components.
Arq is the main way you interact with Arq. You can configure which files are backed up, browse and restore from backups, and change preferences.
Arq Agent is a background app contained within the Arq.app bundle. It shows up as a small icon in the menu bar: (You can hide the icon in the Arq Preferences)
Arq Agent backs up your files to S3.
When you first launch Arq, if Arq Agent is not running Arq launches it.
The first time you run Arq it asks if you would like to add Arq Agent to the "login items" so that backups happen as scheduled. You can also add or remove Arq Agent from "login items" later by toggling the "Start Arq Agent at login" checkbox in the Arq Preferences.