launchctl is a command-line interface to Apple's launchd manager for launch daemons (system-wide services) and launch agents (per-user programs). launchd loads XML-based *.plist files placed in the appropriate locations, and runs the corresponding commands according to their defined schedule.

# To activate a user-specific agent to be loaded into launchd whenever the user logs in:
launchctl load ~/Library/LaunchAgents/my_script.plist

# To activate an agent which requires root privileges to run and/or should be loaded whenever any user logs in (note the absence of ~ in the path):
sudo launchctl load /Library/LaunchAgents/root_scri.plist

# To activate a system-wide daemon to be loaded whenever the system boots up (even if no user logs in):
sudo launchctl load /Library/LaunchDaemons/system_daon.plist

# To show all loaded agents/daemons, with the PID if the process they specify is currently running, and the exit code returned the last time they ran:
launchctl list

# To unload a currently loaded agent, e.g. to make changes (note: the plist file is automatically loaded into launchd after a reboot and/or logging in):
launchctl unload ~/Library/LaunchAgents/my_script.plist

# To manually run a known (loaded) agent/daemon, even if it isn’t the right time (note: this command uses the agent's label, rather than the filename):
launchctl start my_script

# To manually kill the process associated with a known agent/daemon, if it's running:
launchctl stop my_script