.profile all do pretty much the same thing: set up and define functions, variables, and the sorts.
The main difference is that
.bashrc is called at the opening of a non-login but interactive window, and
.bash_profile and the others are called for a login shell. Many people have their
.bash_profile or similar call
Other files of note are:
/etc/profile, for system-wide (not user specific) initialization code.
.bash_logout, triggered when logging out (think cleanup stuff)
.inputrc, similar to
.bashrc but for readline.