typedef struct {
   int data;
   NodeT *next;
} NodeT;

Because NodeT is not yet known (to the compiler) when we try to use it to define the type of the next field.

The following is also illegal in C:

struct node {
   int data;
   struct node recursive;    
};

Because the size of the structure would have to satisfy sizeof(struct node) = sizeof(int) + sizeof(struct node) = ∞.