It's an editor for any .properties file, but if it recognizes a Minecraft file, it will tell you what each option does and what are valid options.
It's written in java and is completely platform-independent.
If you use "open with" on a properties file, it should work, or you can just run the jar.
NO INSTALLATION IS NEEDED, JUST DOWNLOAD AND RUN
About the "Add Rules" button:
This feature allows you to add more custom description and pulldowns for modded server.properties files. They are a custom format (.mcrules). If you want one created, pm (or email) me with a list of the keys, their descriptions, and if any need a custom dropdown box (if they do, i need a list of the values shown to the user paired with their values to be written to the file). I am a server admin myself, but I no longer use modded servers, so I'll need help from you guys for that one.