What is the installer?

[SaberToothedTiger]

When I go to download the game, there's the regular versions, but under that there are ones called KSP Installer. What are they and should I download them?

[TheMoonRover]

The installer is a program which installs current version of the game, presumably in "C:\ProgramFiles(x86)" or an equivalent location on your computer. Many programs have installers which do this. I would imagine it also adds a desktop shortcut and similar things which wouldn't be added if you just downloaded the game directly (although I don't know for sure).

You don't need it. You can just download the version(s) you want and extract them wherever you like from the zip file, then run the game from there. The installer is just an alternative way of getting the game.