The activity page is the default homepage, you can consider it as your control panel about what happens in the site (maybe in addition to the dashboard, even if the dashboard is an option-plugin you can activate or not, whereas the activity page is built into core).

The river provides a number of widgets that display a user and his friends' latest activities. Each entry shows who performed the action and allows you to click through to it and the subject of that action. You have three tabs: "All" shows you yours plus your friends actions; "Mine" and "Friends"... guess which!

An entry is only displayed if an individual plugin has provided information about its activities in a form that the river can understand, and entries are limited to operations that the user has been allowed to see.

A few technical details: the river is not a plugin, it's built into core, and it has its own table in the database. We will see these things further.