Here are the classes, structs, unions and interfaces with brief descriptions:

[detail level 1234]

conf
Elgg	Save menu items
Admin	Upgrade object for upgrades that need to be tracked and listed in the admin area
Ajax
Response	JSON endpoint response
Service	Models the Ajax API service
Amd
Config
ViewFilter
Application
CacheHandler
Database	Elgg 2.0 public database API
ServeFileHandler
Assets
ExternalFiles
Cache
Pool
InMemory	An in-memory implementation of a cache pool
Noop	Implements the caching API but doesn't actually do any caching
StashWrapper	Defers to Stash for the meat of the caching logic
EntityCache	Volatile cache for entities
LRUCache
MetadataCache	In memory cache of known metadata values stored by entity
PluginSettingsCache	In memory cache of (non-user-specific, non-internal) plugin settings
Pool
SimpleCache	WARNING: API IN FLUX
SystemCache
Composer
PostInstall	A composer command handler to run after composer install
PostUpdate	A composer command handler to run after post-package-update event
Core	Activate a plugin or plugins
Database
EntityTable
UserFetchFailureException	Exception indicating a user could not be looked up for a permissions check
AccessCollections
AdminNotices
Annotations
Config
ConfigTable
Datalist
EntityTable
MetadataTable
MetastringsTable
Mutex	WARNING: API IN FLUX
Plugins	Persistent, installation-wide key-value storage
PrivateSettingsTable	Private settings for entities
QueryCounter
RelationshipsTable
SiteSecret
SubtypeTable
UsersTable
Debug
Inspector
ViewComponent
Inspector
Di
DiContainer
FactoryUncallableException
MissingValueException
ServiceProvider
FileService
File	File service
Filesystem
Directory
Fly	A wrapper around Flysystem that implements Elgg's filesystem API
InMemory	Namespace for generating in-memory filesystems
Local	Namespace for generating local filesystems
Directory	A simple directory abstraction
File	Represents a file that may or may not actually exist
MimeTypeDetector	Detect the MIME type of a file
Forms
StickyForms
Http
DatabaseSessionHandler
Input
Request	Elgg HTTP request
WebAppManifestResource
I18n
ArrayMessageBundle	WARNING: API IN FLUX
InvalidLocaleException	Indicated an invalid Locale was given
Locale	WARNING: API IN FLUX
MessageBundle	WARNING: API IN FLUX
MessageTemplate	WARNING: API IN FLUX
MessageTranslator	WARNING: API IN FLUX
NullMessage	WARNING: API IN FLUX
NullTranslator	WARNING: API IN FLUX
ReleaseCleaner	Removes invalid language files from an installation
SprintfMessageTemplate	WARNING: API IN FLUX
Translator	WARNING: API IN FLUX
TranslatorInterface	WARNING: API IN FLUX
Json
EmptyKeyEncoding
Mail
Address	TODO(ewinslow): Contribute something like this back to Zend project
Menu
Menu	A complete menu, sorted, filtered by the "prepare" hook, and split into sections
Service	Methods to construct and prepare menus for rendering
UnpreparedMenu	Linear set of menu items collected from configuration and the "register" hook
Notifications
Event
Elgg_Notifications_Event
Notification
Elgg_Notifications_Notification
NotificationsService
SubscriptionsService
Project
CodeStyle
Queue
DatabaseQueue
MemoryQueue
Queue
Security
Base64Url	Encode and decode Base 64 URL
Hmac	Component for creating HMAC tokens
Services
AjaxResponse	JSON endpoint response
Config	Describes an object that manages Elgg configuration values
Structs
Collection
InMemory	Uses native PHP array to implement the Collection interface
Collection	A read-only interface to a (possibly mutable) group of items
SystemMessages
RegisterSet	Represents the state of system messages and errors
ActionsService
Application
AttributeLoader
AutoloadManager
BootData	Serializable collection of data used to boot Elgg
BootService	Boots Elgg and manages a cache of data needed during boot
ClassLoader
ClassMap
CommitMessage
Config	Access to configuration values
Context	Manages a global stack of strings for sharing information about the current execution context
Database
DeprecationService
DeprecationWrapper
EntityDirLocator
EntityIcon	Entities that support icons should implement this interface
EntityIconService	WARNING: API IN FLUX
EntityPreloader	Preload entities based on properties of fetched objects
EventsService
GroupItemVisibility
HooksRegistrationService
Logger
MethodMatcher	Identify a static/dynamic method callable, even if contains an object to which you don't have a reference
PasswordService
PersistentLoginService
PluginHooksService
Profiler	Analyzes duration of functions, queries, and processes
Router
SystemMessagesService
Timer	Capture timing info for profiling
Translit
UpgradeService	Upgrade service for Elgg
UserCapabilities	WARNING: API IN FLUX
Values	Functions for use as plugin hook/event handlers or other situations where you need a globally accessible callable
ViewsService	WARNING: API IN FLUX
WidgetDefinition	WidgetDefinition
WidgetsService
elgg_rtd_theme
CallException
ClassException
ClassNotFoundException
ConfigurationException
CronException
DatabaseException
DataFormatException
ElggAnnotation
ElggAutoP
ElggBatch
ElggCache
ElggComment
ElggCookie
ElggCrypto
ElggData
ElggDiskFilestore
ElggEntity
ElggExtender
ElggFile
ElggFileCache
ElggFilestore
ElggGroup
ElggIcon	Entity icon class
ElggInstaller
ElggMemcache
ElggMenuBuilder
ElggMenuItem
ElggMetadata
ElggObject
ElggPAM
ElggPlugin
ElggPluginManifest
ElggPluginManifestParser
ElggPluginManifestParser18
ElggPluginPackage
ElggPriorityList
ElggRelationship
ElggRewriteTester
ElggRiverItem
ElggSession
ElggSharedMemoryCache
ElggSite
ElggStaticVariableCache
ElggUpgrade
ElggUser
ElggWidget
ElggXMLElement
Exportable
ExportException
Friendable
Importable
ImportException
IncompleteEntityException
InstallationException
InvalidClassException
InvalidParameterException
IOException
Locatable
Loggable
LoginException
NotificationException
ODD
ODDDocument
ODDEntity
ODDMetaData
ODDRelationship
PluginException
RegistrationException
SecurityException
XmlElement