Packages
com.google.appengine.api.datastore The datastore provides persistent storage for App Engine applications, used either directly or via the provided JDO or JPA interfaces.
com.google.appengine.api.images This package provides facilities for the creation and manipulation of images by Java code.
com.google.appengine.api.labs.taskqueue  
com.google.appengine.api.mail App Engine applications can send email messages on behalf of the app's administrators, or of authenticated users, using the standard javax.mail interface over this low-level API.
com.google.appengine.api.memcache The memcache API provides access to fast but unreliable data storage, which may also be accessed via the JCache API.
com.google.appengine.api.quota  
com.google.appengine.api.urlfetch The URL Fetch service can be used to make HTTP and HTTPS requests of other servers on the internet.
com.google.appengine.api.users The UserService provides facilities to check whether a user has authenticated using their Google Account, retrieve their email address, and check whether they are an administrator for this application.
com.google.appengine.api.xmpp