Internal linking is for when one article refers to another article within the Lexicanum and a direct connection between the two is possible.
Internal linking is the foundation upon which wiki's work for, without it, there would be no way to navigate to related articles. Simply, they are the structure which allows visitors to flow from one article to another. Complexly they are the system's way of maintaining connections between all the articles to form a giant web of information through which visitors pass constantly.
Internal linking is very simple. Placing two square brackets around one or more words will create an internal link. For example [[Dog]] would link this help article to an article named dog.
There are two statuses for internal links to occupy, linked and unlinked. Linked simply means that the connection has been made successfully and will appear link this, while an unlinked link is unsuccessful and will appear like this. Clicking on the unlinked link will allow you to create a new page with the name used in the link, while a linked link will take you straight to the page you chose to link to. Of course linking requires that you know the correct name to link to.
You can chose to have a link go to one article but look like it is going to another. This can be useful when using different spellings or words to represent the same thing. This is achieved by placing a | after the word and then typing the name you would like it to appear as. For example [[Dog|Cat]] would link to Dog, but appear on the article as Cat. Another way of doing this is to use a redirect, but that is more useful in different circumstances.