Installing applications (which roughly equates to "installing packages") is a fundamental part of the Linux experience. A Linux distribution, such as Salix (or Slackware, on which it is based) could be said to consist of its repositories, where the software available for that distribution is kept. These packages have been compiled with the kernel and build tools provided by the distribution and tested to ensure they work together. Salix, like most distros (but not Slackware) provides full dependency management, which means that any items the package needs to run are installed too -- and also that they are guaranteed to be available.
slackbuilds.org (SBo) provides build scripts for further software not present in the repositories. This means the package will be built on your system before being installed. Salix provides a graphical tool, Sourcery, to manage this; its command-line equivalent is slapt-src. Both do have limited dependency management, but the build is not guaranteed to be successful and some manual tinkering may be needed.
Unlike the software provided in the repositories, these scripts are not maintained by either Salix or Slackware but by individual users, and as such are not the responsibility of either distribution, although help may be found on the forum. If the build fails, the last ten lines or so of the output usually provide an indication of the reason. Most commonly, this is a missing dependency, which may be a build dependency (needed to compile the package, but not to run it). Common packages that are required by several SlackBuilds, but not included as part of a standard Salix installation are cmake and linuxdoc-tools, so first make sure you have these installed if you encounter any problems.
If you don't mind using a bit of extra space on your hard drive, then most of these problems could disappear by installing the slackware/d and slackware/l package sets:
sudo slapt-get --install-set slackware/d --install-set slackware/l
You can also create and install your own packages, for which Salix
provides a suite of console scripts, especially
for further information, see the wiki pages on packaging. When
installing packages from any other source, proceed with caution.
In a nutshell, the first place to look for an application you'd like to use is the Salix repositories, which also access the core of Slackware packages around which Salix is built (Gslapt, slapt-get); second port of call should generally be the slackbuild repo (Sourcery, slapt-src). It is not recommended (especially for beginners) to reconfigure the package management tools to access any other repositories. Should you wish to install any such package, it is probably best to download it individually and install manually (for instance with pkgtool or spkg ). Finally, you can make your own package.
Note that twapake is a useful tool available from the repositories to keep track of installed packages, from whatever source.