Function implementing interface

I want to know what is happening here.

There is the interface for a http handler:

type Handler interface {
    ServeHTTP(*Conn, *Request)
}

This implementation I think I understand.

type Counter int

func (ctr *Counter) ServeHTTP(c *http.Conn, req *http.Request) {
    fmt.Fprintf(c, "counter = %d\n", ctr);
    ctr++;
}

From my understanding it is that the type "Counter" implements the interface since it has a method that has the required signature. So far so good. Then this example is given:

func notFound(c *Conn, req *Request) {
    c.SetHeader("Content-Type", "text/plain;", "charset=utf-8");
    c.WriteHeader(StatusNotFound);
    c.WriteString("404 page not found\n");
}

// Now we define a type to implement ServeHTTP:
type HandlerFunc func(*Conn, *Request)
func (f HandlerFunc) ServeHTTP(c *Conn, req *Request) {
    f(c, req) // the receiver's a func; call it
}
// Convert function to attach method, implement the interface:
var Handle404 = HandlerFunc(notFound);

Can somebody elaborate on why or how these various functions fit together?