Hey,
I'm still confused about packages.
When I download for example package which is made for Ubuntu and extract it and manually 'mv' files to correct places e.g. /usr/bin/someprogram, program seems to work just fine.

But what is difference in the end? Is it that different distros might have different folder structures and might have different programs for managing something and around which they are built ?
Programs are same for distor but metadata is different so it can satisfy management programs, slackwares pkgtool needs entries in /var/log/packages so package(s) can be uninstalled in future.

So for example, does slackbuild scripts just extract packages, maybe compile and just put files into slackware like folder structure and create slackware package? (I have read slackbuild script, but didn't understand anything)

Slackware packages are ready-to-install (you use the installpkg utility for initial installation and the upgradepkg utility to upgrade a previously-installed package with a newer version).

SlackBuilds are the directions for constructing an installable/upgradeable package from source code. SlackBuilds are executed to perform the entire process (except for the actual install/upgrade which you do when you're ready to).

When you installed Slackware, you were installing packages built with SlackBuilds. When you download and upgrade packages from the patches directory at a Slackware mirror site, you are upgrading packages built with SlackBuilds.

Your distribution media -- CD-ROM/DVD -- has all the source code for every package (plus some extras in the extra directory on the media. If you look in the source tree and look in the directories located there, you'll find a SlackBuild in every directory containing source code (usually, the source is in a .tar.gz archive (or one of the other commonly-used archive/compression utilities).

The distribution, on the other hand, is the entire system that you installed. Slackware is a distribution as are Ubuntu, Debian, etc. Different distribution take different road to get to the same place (an installed working Linux system); Slackware does that with SlackBuilds.

Hope this helps some.