User Authentication against remote authentication server

sorry, this might be a little bit off the topic, but I really hope you could help anwering this question. It's pretty clear how to use web server to authenticate the users/groups, for example, if we are using Apache, the userID and password will be stored in the browser's cache, and be sent to the web server to do the authentiation whenenver a page is requested. However, if the authentication server is not the web server, instead, it is a remote independent server. How can we manage the user authentications at the web server side? Thank you very much in advance!