This tells you that you have followed that link. Usually links are blue and change to purple once you've clicked on them. This way you know where you've been and don't end up re-visiting the same webpages. The webpages you visit may exist on the same website or they may exist on completely different websites. Links can be text, in which case they usually appear as colored, underlined words or phrases.
Links can appear as buttons or they can be embedded in graphic images.
Often several links are contained in the same graphic. Each webpage has its own unique URL, or uniform resource locator. The three-letter ending of this part designates the type of organization running the host. The ending .edu indicates an educational institution. Pinpoints exactly where on the host computer the document is located.