I like .app a bit more than .pkg, but have no idea whether a .pkg installer without admin rights can actually do anything more than a bunch of shell scripts bef

I like .app a bit more than .pkg, but have no idea whether a .pkg installer without admin rights can actually do anything more than a bunch of shell scripts before and after the installation. Can you link me to an article or a piece of documentation that would explain it to a non-mac-developer?