PDF/X is an umbrella term for several ISO standards that define a subset of the PDF 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-2 and PDF/X-3 accept calibrated RGB and CIELAB colors, while retaining most of the other restrictions of PDF/X-1a.
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 of the BleedBox.