How do we create hyperlinks both to other `items` and `external` content?

What I'm trying to figure out is: how do we link two items?
How do we create a _hyperlink_ to external content?
These might be _similar_ things or _entirely_ different challenges.

Provisionally I'm noting down the following:
+ `link` - a hyperlink
  + `id`: `Int`
  + `url`: `String`
  + `item_id` (FK item.id)

But this is by no means the "solution"! 
Help very much welcome/wanted!