EDI X12 help documentation

Hello,

I want to know whether there is a website available which explains the X12 EDI segments and their field description . For eg BEG segment I know carries Purchase order but there are many segments which I do not know. on X12.org, I cant see anything. Can someone tell me why ?

Just like we have help.sap.com, there must be some website available or some international organisation which publishes EDI X12 standards and the EDI segment meanings.. Googling it, there are many pages from Pvt websites which explain certain segments but not all.

Any pointers will be of great help. A free one would be better.

regards

sachin