When you post something you can either post it to your Site or to the Forums (or both).  If it's on your site it is an article, if its in the forums it is a post. 

We consider items that are on the forums to be more casual and items on your site to be more formal.  So when you are writing on your site most likely you are writing something informational.  But most people who write in the forums are either asking questions or writing something to get an informal discussion going.

So an article is  usually something that is more informational and we try to highlight those items  in a more formal article setting instead of a forum setting.

When you visit the Articles section, the articles that show up in the main list are items that we have featured because we felt that its subject would be of interest to the majority of people who visit the WinCustomize.  And in most cases, the articles are going to be from people who have a WinCustomize Personal Skinning Site or a JoeUser Blog.  On occasion a forum post could be featured and make it into the articles section but most of the time it will be an item that was posted on someones site.

Hopefully I didn't make that too confusing and helped explain it a bit better.