MHEG - (Multimedia and Hypermedia Information Coding Experts Group) MHEG includes an interactive middleware (software) used for digital terrestrial television. MHEG includes an interactive middleware (software) used for digital terrestrial television. "Originally developed and standardized by ISO in the mid 1990s, MHEG-5 is a low-cost open standard that can run on limited hardware. It also has limited capabilities compared to more advanced proprietary platforms, or the MHP standard which was designed as its successor." (Informitv) - http://informitv.com/glossary/mheg & http://www.mheg.org

MHEG-5 - (Multimedia and Hypermedia information coding Experts Group) - The MHEG-5 standard was originally developed a decade ago and offers very basic interactive facilities through a declarative description language. This means that unlike procedural languages which can be used to produce more complex interactive applications, MHEG-5 is generally limited to simple text and graphics services. A working group is currently developing a specification to enable the use of a broadband return path with MHEG-5. (Quote taken from informitv article.) 

www.mheg.org/users/mheg/faq.htm
http://en.wikipedia.org/wiki/MHEG-5
www.yourencyclopedia.net/mheg_5.html

http://informitv.com/articles/2006/10/27/newzealandfreeview

MHEG-3 - MHEG regarding scripting.

MHEG-6 - (Multimedia and Hypermedia information coding Expert Group) - MHEG standards and specifications were developed to increase MHEG-5's performance and access to various external services. MHEG-6 is often more associated with advanced MHEG interactivity.
http://en.wikipedia.org/wiki/Mheg