It's definitely quirky in its inconsistency, but

   type 'a Tree = 
     | Node of ('a Tree, 'a Tree)
     | Leaf of 'a

Is saying "The possible cases for the type describing an 'a Tree are: a Node of two other 'a subtrees, or else a Leaf of an 'a."

Then when you concrete-ize it, you have an "int Tree" or a "string Tree".

This is a little clearer with thinking of the List datatype: this is a number list, that's a string list, that's a grocery list, this is a guest list...etc.

One nice thing in F# is that there's an ability to "standardize" how you write generics to look more like the (admittedly arbitrary) way that C# or Java or C++ write them:

    type Tree<'a> =
      | Node of (Tree<'a>, Tree<'a>)
      | Leaf of 'a