This is probably a programming nightmare for someone like me , but how difficult is it to create a backdoor to edit a website? I know there are systems like Wordpress, but how is that created for websites built using programs on a computer? I made a website for someone, and a programmer friend of mine built a backdoor admin for it. It was really great, but he couldn't tell me how he did it - not in a way that made sense to me.

There are apps you can use to do this, one of them is called Cushy CMS which you can find more information on here:
http://www.cushycms.com/en

I'm sure there are others too, but this is the only one I can remember at the moment.

Another one is http://www.snippetmaster.com/. It costs a wee sum, but it easy to use, and you can assign editor rights to whoever needs it.

There is also a free version, but if you have that, you'll not be able to upload anything online.