Well, "reviewed by" is not one of the link types JIRA ships with, so your admins have added it, and hence it's probably something that needs documenting for your users by your organisation!
Link types really are just a list for categorising the links between issues according to how you want to do it. Admins can add or remove any link type. But there's no difference between them really, it's just a label for you to define and use. The only link type that does anything off-the-shelf in JIRA is when you clone an issue, the new issue will be linked back to it's source.
Some addons do make use of links though. Agile uses an "Epic Link" (and also separates it out from the others). I think there are a couple of testing related tools that add their own link types for you and then use them to link things together and report on them. I've written addons that use links to define relationships between components. And so on.
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