Objects are the entities that are passed by apps to the Flock platform via Methods, and can be passed to apps by Flock via Events. There are currently 5 different types of objects that an app can use. 

MessageThe Message object is used to send messages between users and groups, and which will appear within the chat window.
AttachmentAn attachment object is included in any message that has rich content. Attachments are powerful in that they allow you to set multiple properties including title, description, widget, FlockML content, downloads, and attachment buttons.
UserAn individual user who has an account on Flock.
Group A collection of two or more users in Flock.
PublicProfilePublic profile of a Flock user.