the standard in question is ISO 3166.

ccTLDs don't correspond directly to the codes in ISO 3166, since a registrar must petition ICANN to add it to the root zone. however, the code basically has to be in ISO 3166 for ICANN to approve it as a ccTLD, so keeping the whole ISO 3166 list around would be a good way to definitively look up any possible ccTLD.

also, that standard does change a few times every couple of years. for instance, Western Samoa (WS) was recently changed from "Independent State of Western Samoa" to "Independent State of Samoa". more recently, "Occupied Palistinian Territory" (PS) was added (it actually also was subsequently granted a ccTLD which was added to the root.zone within the last few months).