There are many different objects that are specific. Understanding how they are related and what is inherited from what may help to understand how they function and the purpose they server. You can read about each of them on their respective pages. Here is the class hierarchy:
Component
Sensor
Service
Actuator
ComponentManager
Connector
Device
SICRedis
Message
Request
StartComponentRequest
CompressedImageRequest
TextRequest
ControlRequest
ConnectRequest
PingRequest
StopRequest
NotStartedMessage
ConfMessage
AudioMessage
TextMessage
CompressedImageMessage
UncompressedImageMessage
BoundingBoxesMessage
ControlMessage
PongMessage
SuccessMessage
IgnoreRequestMessage