Technically, there's not any difference. It should point to the same page in most cases. This is called the absolute path versus the relative path to a file.
For optimization purposes, it's better to specify the full url in all your links, even if it's a path to an image. There are many reasons why, including:
- to prevent domain hijacking
- to prevent indexing problems (if you have more than one domain pointing to content then it can get really messy really quickly)
- to prevent an accidental change in PageRank being applied to the wrong page. (domain.com/index.html is different than
www.domain.com/index.html).
- to prevent dns hijacking of a site
- to prevent pagejacking
Whatever you do, be consistent. If you use relative paths then use it throughout the site. I recommend, though, that you always specify the full path of the url.