Slackware is Linux. What "is" UNIX is tricky. Have a peek at
What is UNIX ?
if you are interested.
Most Linux and *BSD distributions provide some form of 'net boot' ISO which allows you to down load a minimally sized ISO which contains enough information to boot the machine and then uses a network connection to pull the rest of the material needed for the install over the net. I did a bit of poking, and it doesn't appear that Slackware has such a thing as a net-install ISO.
Maybe someone knows differently, but it wasn't obvious to me.