// *** before: ***
uses
  SysUtils, Classes, Forms, Graphics;

// unit names were flat and global: a unit of yours named Types or
// Classes collided with the one in the library

// *** in version XE2: ***
uses
  System.SysUtils, System.Classes,   // the language and the RTL
  Vcl.Forms, Vcl.Graphics,           // the Windows-only library
  FMX.Forms;                         // the cross-platform library

// the prefix says where a unit comes from: System, Vcl, FMX, Data, Web,
// Xml, Soap. Short names still compile: the project keeps a list of unit
// scope names, and the compiler tries them as prefixes one by one