They are not the same thing. I guess you mean the cli command
/etc/inittab is a file which is read by the init process on startup. Thats the process that takes care of starting and stopping services inside your system.
chkconfig is a cli tool that create and remove filesystem symlinks. these symlinks are used to tell the init process at which stage of the system boot, which process should be started or stopped.
man inittab and
man chkconfig should tell you more details about that
man 8 init you can also see a description of what the
init process is actually using the