Besides the login items Bob mentioned, there are at least two other areas of the OS that will cause apps to launch the next time you turn your Mac on, or restart it.
Choose Shut Down or Restart from the Apple menu at the top left. Doesn't matter which one. Uncheck the box to reopen windows. Then click Cancel. This box will now stay off.

Open the System Preferences. Under the General tab, turn the check box on to close windows when quitting an app. Any window that is open for an app when the Mac is shut down or restarted will cause that app to automatically launch on the next startup or restart.
