A single web browser window on your desktop may contain several tabs. Each tab is an independent browsing context. Each has its own Window object, and each is isolated from all the others. The scripts running in one tab usually have no way of even knowing that the other tabs exist, much less of interacting with their Window objects or manipulating their document content. If you use a web browser that does not support tabs, or if you have tabs turned off, you may have many web browser windows open on your desktop at one time. As with tabs, each desktop window has its own Window object, and each is usually independent of and isolated from all of the others.
But windows are not always isolated from one another. A script in one window or tab can open new windows or tabs, and when a script does this, the windows can interact with one another and with one another’s documents (subject to the constraints of the same-origin policy of The Same-Origin Policy). Opening and Closing Windows has more about opening and closing windows.
HTML documents may contain nested documents using an
<iframe> element. An
<iframe> creates a nested browsing
context represented by a Window object of its own. The deprecated
<frame> elements also create nested
browsing contexts, and each