Some applications are designed to start as soon as the phone or tablet boots up. Some of these toggle this via settings, but others are designed to start automatically and can't (easily) be prevented from doing so.
Moreover, there are many other "events" which can trigger apps to be started automatically (e.g. "SDCard mounted", "SMS received", "Network status changed"). So whenever your issue is about an app starting automatically without your triggering its start, this tag is the right one.
There are tags related to this issue -- you might e.g. wish a given app should start automatically while it doesn't do so:
- automation: Trigger actions to be automatically executed on specific events, such as e.g. time, location, key-presses -- or even sensors, connected networks, etc.
- startup: Other things related to the startup process of your device (except from apps starting automatically after booting, which is what auto-start is for)