An input session (session) receives one specific type of input events. Unlike windows, sessions don't produce any graphical content and can't be placed into groups. The only grouping that could be considered for a session is the one that's been established for the window that the session is associated with.
Sessions allow applications to control who gets what types of input events and how these input events are delivered. For each type of input event, there is a corresponding session type.
For more information, see the Sessions chapter of the Screen Developer's Guide.