PDF/X is a subset of the PDF ISO standard. The purpose of PDF/X is to facilitate graphics exchange, and it therefore has a series of printing related requirements which do not apply to standard PDF files. For example, in PDF/X-1a all fonts need to be embedded and all images need to be CMYK or spot colors. PDF/X-3 accepts calibrated RGB and CIELAB colors, while retaining most of the other restrictions of PDF/X-1a.
PDF/X files must not only follow certain restrictions, they also must contain a special file identification, inside the PDF, which says which PDF/X version they are. This means that a file can only conform to a single specific PDF/X standard, even if all other requirements are met.
The printing conditions or output intent need to be specified in the file. This can be specified in the form of standard profiles using codes, like "CGATS TR 001 SWOP".
In a PDF/X file that has color managed data each color managed graphic gets its own color profile, so even though the file as a whole is CMYK, individual graphics may be RGB (with calibration information).
Various boxes must be defined. The MediaBox which defines the size of the entire document, either the ArtBox or the TrimBox, which define the extent of the printable area. If the file is to be printed with bleed, a BleedBox, which must be larger than the TrimBox/ArtBox, but smaller than the MediaBox, must be defined.
Active content is not allowed in a PDF/X file. This means that standard PDF features like forms, signatures, comments and embedded sounds and movies are not allowed in PDF/X. Features that are forbidden in the PDF/X standard can sometimes be used, if they do not affect the rendering of the file. This allows for things like annotations outside the BleedBox.
PDF/X-6 is in development which will be the new print production standard built upon PDF 2.0.
PDF/X is formalized in ISO standards 15929 and 15930:
All versions of PDF/X contain file identifications which indicate the version of PDF/X to which they conform. This appears in a document info dictionary, in document metadata, or both, according to the standard used. As of 2010, the complete list of file identifications are the following strings: PDF/X-1:2001, PDF/X-1a:2001, PDF/X-1a:2003, PDF/X-3:2002, PDF/X-3:2003, PDF/X-4, PDF/X-4p, PDF/X-5g, PDF/X-5pg. Observe that this means that the various revisions of PDF/X-1a and PDF/X-3 are distinct types, while the revisions to PDF/X-4 and PDF/X-5 variations are not.