The NCIP standard/protocol is used primarily in the US and Canada, and is the standard that is used to exchange circulation information between systems such as self-service circulation, ILL systems, and other systems that need to pass/exchange data about item availability and other circulation type information.
The Z39.88 standard, or Open URL Framework for context sensitive services. Open URL is used heavily to connect different services related to discovery and delivery. Open URL allows users to navigate from a citation and either be direct to a full version or request a full version in a fulfillment system.
Digital Object Identifiers are standards for unique identifiers for digital objects, and is an ISO standard: ISO 26324. The DOI is used primarily to identify articles from journals, book chapters, books, and other objects.