What's the difference between this code?

I was wondering the difference between Function and Method when used in the following specific manner. I can't really see a difference because they both work the same when used like this.

Type Blah
   Field a:Int

   Function Add(this:Blah)
      self.a :+ this.a
   End Function
end type

and

Type Blah
   Field a:Int

   Method Add(this:Blah)
      self.a :+ this.a
   End Method
end type