URL = Uniform Resource Locator. Its the fancy name for a web address.
www.fordsix.com is the URL of the Ford Six Website.
HTML = Hypertext Markup Language (I believe). Its a basic programming language that provides the formatting for most, if not all, of the stuff you see on your screen.
To add your own avatar, you have to have the picture hosted on the web somewhere. You cant just link to a picture on your hard drive, because it will be unavailable to everyone who tries to view it (thats a good thing, btw).
It looks like you can upload a picture to the gallery at fordsix.com, but you can also upload a picture free to sites like
http://photobucket.com/. If you want to use that picture as an avatar on multiple websites, you should probably use
http://photobucket.com.
Once you have the picture hosted on the web, go and view it and copy down the web address (URL). Then come back to the forums here and click on the "profile" link at the top of the page and paste the URL into the appropriate field for avatar. If you uploaded the pic to the gallery here, you can just click "browse gallery" and find the pic you want. Also, make sure to read the avatar size restrictions listed under the profile link.
Hope that helps.
-Andrew
