Links are small pieces of code, with which the writer gives the user the Choice to move from one document to another. These links need not be to the same site, but can incorporate whole other sites or just individual pages that might further explain the topic in hand.
Links are the tools with which a hypertext document writer can allow a user to go away and look at other related topics, or to expand a subject to a higher level of information. Enabling the reader to take on as much or as little as they require.