ConnectApi is the Apex namespace that includes Chatter in Apex functionality.
Any questions about the methods in the ConnectApi Apex namespace should have the connect-api tag. The ConnectApi classes and methods are referred to as "Chatter in Apex" and contain most of the functionality offered in the Chatter REST API. A primary use case is to create customized Chatter UIs in VisualForce.
In the Apex documentation, you'll find information about Chatter in Apex in two main places: