When you login with a new shell, bash reads /etc/profile and /home/user/.bash_profile if it exists.
Slackware doesn't have a .bash_profile in the user's directory by default, but just create it with the aliases, functions, or commands you want to run on login and bash will find and use it.
You can also invoke, or start, bash as a subshell -- meaning it's not a log-in, just another process running under your current shell. When bash starts in this manner, it doesn't read the profiles... It looks for /etc/bashrc or /home/user/.bashrc
A simple way to get bash to read the log-in commands, aliases, etc every time a new instance is started is to keep all of your customizations in /home/user/.bash_profile and then simply have these two lines in your .bashrc:
Thus, .bash_profile is read on login, and from a subshell, as referenced through .bashrc
Also, on another note, Slackware uses BSD style initscripts, rather than the System V style most other linux distrobutions use. The layout of the /etc/rc.d/ directory is a bit different, but in my opinion, more straightforward.
There is a HOW-TO at www.tldp.org
called "From Boot to Bash" which goes over the whole boot process, if you're interested. There may also be a bit on this at www.freebsd.org
in the freebsd handbook, which is also an excellent source of dry, easy to follow information.