Pages are plain modules with nothing special that display output.

Snapins are full on plugins which inject resources into the webserver. WebSockets is a good example of this which injects both a protocol transition handler and a websocket handler into the webserver.

The statistics plugin/interface is a system which allows for a central statistics management system. This system is responsible for storing of statistics, providing a subscription interface which can trigger events on new statistic information. This interface is fed from a specific shaper statistics module by triggering its statistics update event. Look at the tcstats plugin for an example.