Creating Photoshop Plugin

I want to create a Photoshop plug in. I recently downloaded an ICO plug in (iconformat.8bi) that saves the file as an icon. I thought it was just psp's action script file with the extension changed. But when I opened it in notepad I found that it was full of garbage with the words "this program cannot be opened in DOS mode".
So it's a program... I googled for 'creating adobe Photoshop plug in'. But it returns tons of psp tutorial sites where you can download plug-ins; NOT create them.
Does someone know where I can find the specifications for the structure of the program or a library (like scrnsave.lib for making screen savers the easy way), which helps you make it?

P.S.
My version of Photoshop is an old 7 and adobe's site is filled with CS thingies.
The Photoshop help file says that in order to create a plug-in module, I have to contact adobe developer support (I'm not intending to do this professionally or any thing. just to create a simple raw file format for one of my apps, so that I can paint and directly save to that format from psp, and may be actually convert some of my fractal generating programs into plug-ins just for fun).

I'm pretty sure you have to apply on the Adobe website somewhere. You tell them what you intend to make and if they think it's worthwhile (or something) then they give you access to the SDK.

I'm pretty sure this is the route to take even for hobbyist plugins. I already have. Haven't written anything for it yet but my access is there for when I want to. Just another side project waiting to be started.

Well in that case, it might not be worth the trouble {YET}
[Edit]Saw that link[/edit]