SIC consists of many different objects. You can read about each of them on their respective pages. This is the inheritance 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