MPEG-2 Standard

The MPEG-2 Standard is defined in ISO-13818, which is comprised of the following documents (some of them are also published by ITU):

ISO-13818-1 aka ITU-T H.222.0

Systems (definition of the MPEG-2 Stream Formats, the decoder model and the timing/synchronization model)

ISO-13818-4

Conformance testing (read if you want to write a multiplexer or an encoder)

ISO-13818-6

Extensions for DSM-CC (Digital Storage Media Command and Control). The DSM-CC specification covers several different aspects of broadband service delivery, including that addressed by the "Download" Protocol. The Download Protocol, in turn, contains several variations, including the DSM-CC Object Carousel, which is used to transmit interactive multimedia information, such as found in the case with MHP or MHEG-5 applications, as well as other data.

For a full list of the MPEG-2 standard's documents, see the ISO website (link provided below).

Also See

Given that the MPEG-2 Standard is a superset of the MPEG-1 Standard, you will find that it will in places (particularly in the case of audio coding discussions) refer to the older MPEG-1 standards documents, so you may wish to read ISO-11172 as well
The DVB Standards
The ATSC Standards

You can buy all of the above documents from the ISO website; however they are quite expensive.
ITU, on the other hand, comes to the rescue by permitting any individual to (after registration) download up to three documents for free.
Wikipedia's MPEG-2 article
Wikipedia's MPEG article