Link tracking shows you which links in your campaigns and automation emails are being clicked, the number of times those links are clicked, and which contacts clicked them. This is automatically enabled for any campaign or automation email that contains links. You can read more about link tracking here.
Site tracking shows you which contacts visited a specific page on your site and requires that you whitelist a domain and add a snippet of code to your site.
How they work together
Once you have set up site tracking, one of the ways you can identify your contacts is when they click on a link in a campaign or automation email that takes them to a page with site tracking code installed. If link tracking was manually disabled, then we will not be able to identify that contact or track that page visit.
You can read more about how to set up site tracking and the other ways we identify contacts here.