The slackpacks are basically compiled from source. With the ap "checkinstall it's a matter of ./configure && make && checkinstall and you'll end up with a slackpack (.tgz)
The slackware ap "pkgtool" can be used to view a list of all installed packages, install packages, and romove installed packages. There are other tools you can also use as well.
You can also use the command "rpm2tgz" to convert a generic rpm to a slackpack.
You have to manage your own dependency issues but I've never found that to be a draw back and I've compiled allot of arcade type games for my daughter.
It took a while for me to get the hang of it but now I don't put anything in unless its a slackpack. Something like acroread or openoffice which are in my opinion reasonable exceptions to the rule go into my boxes with their own install scripts.
There's allot of packages available other than off the slack ftp sites check out www.linuxpackages.net
if your curious.