I have used Windows XP Embedded, Windows XP Embedded with Service Pack 2, and the re-branded Service pack 3 version of the product, "Windows Embedded Standard" or sometimes "Windows Embedded 2009" or "Windows Embedded Standard 2009" (this is when the namings of the embedded operating systems become much more confusing).
Windows XP Embedded (and its variants at other Service Pack levels) is a software suite that builds an instance of Windows XP using the many sets of components (organized using SQL database). The components are Windows XP components that come with Windows XP Professional plus other sets of components that do not come with Windows XP (such components are or were, however, available on the Microsoft website in various places for download).
I believe that instances of Windows XP Embedded could all the components of Windows XP Professional (I, however, would use a less components) as well as just the bare components for Windows XP to run.
The updates specific to Windows XP Embedded typically address issues that do not manifest in the other version of Windows XP (typically licensing files bugs).
I have not used the "Windows Embedded POSReady 2009" variant, but I expect that it is a re-branded version of Windows XP Embedded with predefined sets of components that are thought to be used at a point-of-sale. Also, from looking at the pictures provided by tomasz86, it seems that the POSReady version comes with an installer that may fully install the Windows XP components. Windows XP Embedded distribution was mostly manually installed, with only an initialization at first boot (which is probably where the REGSVR32 entries are run and the Windows Registration files are merged). The POSReady version seems to fully install the instance of Windows XP.
This post has been edited by Ascii2: 02 September 2012 - 02:18 PM