The idea of
runlevels in
init is found in
System V, one of the two chief
genera of
Unix. The other genus,
BSD, uses a much simpler
init process. The BSD
init program sets the
kernel's
securelevel, runs the boot script
/etc/rc, and maintains
getty processes for the terminals specified in
/etc/ttys. And ... well ...
that's about it.
Linux, incidentally, uses a SysV-style init, but with a different set of runlevels than RancidPickle gives. Notably, runlevel 5 is used for X11 mode, in which console logins are managed by xdm (or a variant such as gdm or kdm). Debian uses runlevel 3 for normal multi-user text console mode; some other distributions use runlevel 2 for this.