A pidgin is a reduced language created when two or more groups with distinct languages are forced into contact - the pidgin emerges in order to provide some level of cross-group communication. Each group retains its own language for "internal" communications but uses the pidgin for "external" communication. One example is a pidgin known as Russonorsk that emerged between Norwegian and Russian fishermen who encountered each other in the Arctic waters they trawled.
Pidgins often arise as a second language for colonists and workers who speak differeing native languages and yet need to talk to each other. For various reasons, the groups are unwilling or unable to learn the language of the other and so fall back on a pidgin as an alternative. For instance, when the English set up a trading post in Canton in the 17th century, both groups had such a high opinion of their own culture and language, and a low opinion of those of the other, that neither community would contemplate learning and using the other language.
Compared to normal languages, pidgins are severely limited in the complexity of communication they can provide. The sounds of a pidgin are generally just those common to the languages that go into it. A pidgin's words generally just consist of nouns, verbs, and adjectives, with little or none of the other components of a full grammar, e.g. adverbs and prepositions.
Pidgins can evolve into more full-featured languages. A creole is the result when a generation of pidgin-speakers begin to adapt that pidgin as their native language rather than a secondary option. Compared to pidgins, creoles have larger vocabularies and much more complex grammars. Consequently, creoles are far more able to support the expression of complex concepts and phrases expected of a real language - those which pidgins can't handle. Creoles often emerge amongst the children of pidgin speakers - it's as if the children naturally recognize the limitations of their parent's pidgin and fill the gaps in order to enable more meaningful communication amongst themselves.
Both Yadis and Web Single Sign On Metadata Exchange Protocol allow a provider to query the identity suites supported by some other, Yadis for URI-based systems like OpenId and LID, WSSOMEX between WS-Federation and Liberty ID-FF 1.2 enabled sites. In the absence of a single URI-based protocol, or in the absence of a single XML-based protocol, both Yadis and WSSOMEX enable a sort of interoperability by answering the question 'What does the other guy speak?'.
Both feel like pidgins in their limited scope and expressiveness, rather than a full-featured creole shared between the two worlds. It's like the previously mentioned fishermen using a pidgin (part of neither language) in order to determine which full language (either Russian or Norwegian) should be used in any particular boat-to-boat interaction.
Lars: Hey Dere Bro
Yuri: Hey Dere Youself Bro
Lars: Me Ken Norse and Russki
Yuri: Dat Good, We Be Using Russki
Also like a pidgin, the motivation for such metadata-exchange is primarily driven by the unwillingness of both sides to deprecate their chosen language in favour of the other.
Hawaiian pidgin offers one very useful construct. 'Da Kine' is a universal term used whenever the speaker can't rememember an actual term or phrase they wish to use. Imagine the usefulness of some universally recognized identity URI for 'interpret from surrounding context'.