small and capital files under /etc/rc0.d/ /etc/rc1.d/ /etc/rc2.d/ /etc/rc3.d/

This is the so-called "System V" or "SysV" boot method.

K means "this program is going to be KILLED" the numbers give the order; first 01 then 02 then the next and so on.

S means "this program is going to be STARTED" and it starts the 01 then 02 and so on.

_ALL_ of these symlinks get their turn in that runlevel (rc3.d = runlevel 3)

Note: 
only CAPITALS get their turn; small letters (s and k as opposed to S and K) don't do anything.