There are two ways of specifying URLs: absolute and relative.
Absolute URLs provide the entire path to a file, such as:
http://www.grantcom.us/images/logo.gif
Relative URLs assume the context of the current file location.
For example, to refer to a URL of a subfolder images containing a file logo.gif, the relative URL would be:
images/logo.gif
Most web sites are built using relative URLs, which is safe as long as the directory structure of that site remains unchanged.