What Abstract Data Structure is this?

What this definition means is that $X$ is either empty or contains some value of typeclass $a$ and two $X$ 's. This is what exactly a binary tree is!

Here is how someone would use this defintion to represent a tree like this:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23

freeTree :: X Char  
freeTree =   
    Node 'P'  
        (Node 'O'  
            (Node 'L'  
                (Node 'N' Nil Nil)  
                (Node 'T' Nil Nil)  
            )  
            (Node 'Y'  
                (Node 'S' Nil Nil)  
                (Node 'A' Nil Nil)  
            )  
        )  
        (Node 'L'  
            (Node 'W'  
                (Node 'C' Nil Nil)  
                (Node 'R' Nil Nil)  
            )  
            (Node 'A'  
                (Node 'A' Nil Nil)  
                (Node 'C' Nil Nil)  
            )  
        )  

Graphics from: Learn You a Haskell for great good