In the directory /etc you'll find a bunch of sub directories starting with rc. The numbers represent the so called run level. Default runlevel may vary per Linux variant. There's also a /etc/rc.local file, that's where I add my local commands, if you need it executed for all run levels.
E.g. runlevel 1 is single user mode, no network, no mounted filesystems, only console. Sort of emergency level. Nowadays nobody cares much about the runlevels, everything is started. In the rc<0-6>.d directories, there are README files with more info.