It differs from portpkg, in fact it is meant to be used with repositories like this:
every SlackBuild is in a different directory and there is a FILELIST.TXT or FILE_LIST that lists all the files contained.
So you can use slackbuilder with official Slackware sources.
Besides slackbuilder supports dependencies: if a file called SOURCES.TXT is present in the repository then slackbuilder reads dependency informations from it. The repository maintainer can create that file executing repos.source, a simple script provided together with slackbuilder: repos.source reads actual dependencies from the slack-required files if they are present in the same directory of the SlackBuilds.
slackbuilder only downloads the files related to the single package you want to build, so you don't have to maintain large ports tree on your hard disk and and --update only takes few seconds.
slackbuilder permits to better manage source repositories: if you put a file called sources that contains a link to the real software sources in the same directory of the SlackBuild, slackbuilder directly downloads the sources from the link you provided. In this way you don't have to put all the tar.gz and tar.bz2 on your repository.
It's distributed under the GPL and developed in bash.
SlackBuilder 0.5.2 sources
SlackBuilder 0.5.2 Slackware package