The pkgtools (installpkg, upgradepkg, removepkg, etc.) make up the official package management suite. slackpkg is a front end that is supported and included in the base install (adding on niceties like network installation). It is hard to think of an Arch equivalent but in the Debian world I guess dselect/dpkg = pkgtools and apt-get = slackpkg.
swaret is an alternative (to slackpg) front end, with no official support in Slackware (it isn't included). I'm not sure that it is all that popular these days (in fact I am not even certain if it is actively maintained). Personally, I'd give it a miss.
Sbopkg is an unofficial (and hence unsupported) front end to Slackbuilds, similar to how Yaourt (and other similar utils) provides seamless access to the AUR. It is very popular and I would certainly recommend it myself!