Link tracking shows you which hyperlinks in your campaigns and automation emails have been clicked, how many times this link has been clicked, and which contacts performed the click. This is a functionality that is enabled by default for every campaign and automation email that contains a hyperlink. You can read more information about link tracking here.
Site tracking shows you which contacts have visited a specific page on your website. This requires that your website domain is on the 'trusted list' and that a tracking code has been added to your website. You can read more about site tracking and how to set it up here.
How do these forms of tracking work together?
If you have set up site tracking, you can identify your contacts when they click on a hyperlink in a campaign or automation email that directs them to a page where site tracking is activated. When link tracking is manually disabled, you cannot identify that contact or track that page visit.