Saber,
Relative paths are "relative" to the document root of the server. i. e. ../mypix/vacationpix/bermuda.jpg. They are easy to move from server to server. But, if you copy that page to anyplace else, ../mypix/vacationpix/bermuda.jpg won't exist unless you used something like webcopy to move all the directives, recursively.
Absolute references have the domain and server path in the URL, i. e. http://www.mydomain.com/mypix/vacationpix/bermuda.jpg.
They will basically work from anywhere.