It depends on how you install your software. As stated below, you can use "installpkg" to install a slackware specefic filename.tgz file and files will go in different places. Configuration files may end up in /etc/ and binaries usually are in /bin/ /sbin/ /usr/bin/ /usr/sbin /usr/local/bin /usr/local/sbin/ etc...it all depends on the software really.
If you compile from source (the best way to install software IMO) then
I like to place all my installed software in /usr/,
read up on installpkg man page to learn how to pass some arguments to installpkg to allow it to show you where the files would be installed. Kind of like a test, it does not run the installation, but tells you what will go where.
also read up on linux partitioning and how it works, and how you should
lay out your system (/usr for software, /boot for kernel, /var for email and logging, /tmp for temporary files, /home for user home directores ) etc.
I like to keep the sources because if you need to recomplie your software to add a new feature, the source is already there and you dont have to go and download it again.