ABCD stands for Access to Biological Collection Data and is an XML based format. It is a ratified TDWG (Biodiversity Information Standards, formerly Taxonomic Databases Working Group) standard and the current version is 2.06b (though 2.06 is most commonly used).
The ABCD documentation can be found on the TDWG Wiki: http://wiki.tdwg.org/twiki/bin/view/ABCD/WebHome
There are over 1400 different concepts that can be expressed in ABCD of which 1100 can contain data.
ABCD allows for extension to the schema for special purposes that are not covered by ABCD. The two most notable extensions are ABCDDNA and ABCDEFG.
ABCDDNA is the ABCD extension for DNA data and is also a ratified TDWG standard. It is maintained by the DNA Bank Network.
ABCDEFG is the ABCD Extension For Geosciences and it is maintained by the GeoCASe Project.
ABCD files can be created using the BioCASe Provider Software (it also works for the DNA and EFG extensions).
A sample document can be found in the documentation wiki for the BioCASe Provider Software: http://wiki.bgbm.org/bps/index.php/SampleABCDDocument
The XML namespace for ABCD 2.06 is
The XML namespace for elements from the DNA extension is
The XML namespace for elements from the EFG is