This is an example of a transparent GIF. No action has been applied to it. You will notice that the art allows the background to show through the parts of the text that would be transparent if it were printed. GIF files are generally great for showing text because they define edges and shapes a little better than do JPGs.
Below is an example of a GIF replaced by a rollover. The first GIF - the one you see below - is blue. It is a pretty standard color for a hyperlink. The rollover that replaces it is red. This is a fairly standard color for an activated link. In any event, the change in colors serves to let users know they have positioned their cursors correctly in order to activate the link.
Even though it looks like text,it is not. It may be more proper to think of it as a picture of text. In this particular image, the background is transparent; no bounding box appears around it. Therefore, the gray background of this page appears within the parts of the text to which no color is applied.
By the way, the link attached to it is one of the better places on the web for describing the basics about web design and how the web works. Remember, GIFs are very handy for preserving edges of graphics. The text here is a good example.