If you want to be notified when Google finds new or changed content on a topic, you can set up a Google Alert. This is handy, for example, if you want to gather information on a particular endangered language.
The service used to be offered right on the Google search page, but that no longer seems to be the case. To set up an alert request, go to the Google Alerts page.
Wikipedia also has a page on this service at Google Alerts, where it says this is a beta service open to the public.