How do you make "plugins" for Unity?

So one day I saw a project called Tiles or something, I'm not sure. In the forums. It was an interesting project, it was like a plugin for Unity. It allowed you to draw 2d, platformer maps.

It seemed to be a completely separate map editor, inside Unity. You clicked, and you placed a tile. You could drag the mouse, copy many tiles, paste them, delete, etc.

I wonder, how do you make plugins like that? They sound interesting and I'm curious on how they work, thanks =D