The Salesforce application supports the use of namespace prefixes. Namespace prefixes are used in managed Force.com AppExchange packages to differentiate custom object and field names from those in use by other organizations. After a developer registers a globally unique namespace prefix and registers it with AppExchange registry, external references to custom object and field names in the developer's managed packages take on the following long format:
namespace_prefix__obj_or_field_name__c
Because these fully-qualified names can be onerous to update in working SOQL statements, SOSL statements, and Apex once a class is marked as “managed,” Apex supports a default namespace for schema names. When looking at identifiers, the parser considers the namespace of the current object and then assumes that it is the namespace of all other objects and fields unless otherwise specified. Consequently, a stored class should refer to custom object and field names directly (using obj_or_field_name__c) for those objects that are defined within its same application namespace.
To invoke a method that is defined in a managed package, Apex allows fully-qualified identifiers of the form:
namespace_prefix.class.method(args)