Service classes - inheriting from Zend_Gdata_App. These also include other classes such as Zend_Gdata, Zend_Gdata_Spreadsheets, etc. These classes enable interacting with APP or GData services and provide the ability to retrieve feeds, retrieve entries, post entries, update entries and delete entries.

Query classes - inheriting from Zend_Gdata_Query. These also include other classes for specific services, such as Zend_Gdata_Spreadsheets_ListQuery and Zend_Gdata_Spreadsheets_CellQuery. Query classes provide methods used to construct a query for data to be retrieved from GData services. Methods include getters and setters like setUpdatedMin(), setStartIndex(), and getPublishedMin(). The query classes also have a method to generate a URL representing the constructed query -- getQueryUrl(). Alternatively, the query string component of the URL can be retrieved used the getQueryString() method.

Feed classes - inheriting from Zend_Gdata_App_Feed. These also include other classes such as Zend_Gdata_Feed, Zend_Gdata_Spreadsheets_SpreadsheetFeed, and Zend_Gdata_Spreadsheets_ListFeed. These classes represent feeds of entries retrieved from services. They are primarily used to retrieve data returned from services.

Entry classes - inheriting from Zend_Gdata_App_Entry. These also include other classes such as Zend_Gdata_Entry, and Zend_Gdata_Spreadsheets_ListEntry. These classes represent entries retrieved from services or used for constructing data to send to services. In addition to being able to set the properties of an entry (such as the spreadsheet cell value), you can use an entry object to send update or delete requests to a service. For example, you can call $entry->save() to save changes made to an entry back to service from which the entry initiated, or $entry->delete() to delete an entry from the server.

Other Data model classes - inheriting from Zend_Gdata_App_Extension. These include classes such as Zend_Gdata_App_Extension_Title (representing the atom:title XML element), Zend_Gdata_Extension_When (representing the gd:when XML element used by the GData Event "Kind"), and Zend_Gdata_Extension_Cell (representing the gs:cell XML element used by Google Spreadsheets). These classes are used purely to store the data retrieved back from services and for constructing data to be sent to services. These include getters and setters such as setText() to set the child text node of an element, getText() to retrieve the text node of an element, getStartTime() to retrieve the start time attribute of a When element, and other similiar methods. The data model classes also include methods such as getDOM() to retrieve a DOM representation of the element and all children and transferFromDOM() to construct a data model representation of a DOM tree.