The X.  and the  V.  Standards

The international  CCITT ("Cunsultative Committee for Telegraphy and Telephony" - since renamed to ITU-T) created all those X. and V. standards that you keep seeing.

The X Standards (X.21, X.25, X.121, etc)

http://www.itu.int/rec/recommendation.asp?type=products&parent=T-REC-x

The V Standards (V.32, V.90, V.35, etc)

http://www.itu.int/rec/recommendation.asp?type=products&parent=T-REC-v

NOTE1: if you must get the details, click on the link for that particular standard, and if it is still active, you can buy a PDF file describing it

Other CCITT Standards (such as R. etc) 

You can use the given link and replace the letter at the end for other standards/  For example, there is an entire set of "R" standards, and the link would be the same as for X, except replace the X with R :

http://www.itu.int/rec/recommendation.asp?type=products&parent=T-REC-R