CompuServe (whose formal name is CompuServe Information Services, or CIS for short and sometimes CI$ for a statement) is a commercial online service that you can access through your modem. It has an incredible array of forums (sections devoted to particular interests), conferences, facilities for uploading and downloading information, and all the other benefits a powerful online service offers.
CompuServe has been around for quite some time now and it's big. Vast. Cavernous. And getting bigger every day. Navigating around the many forums can be quite intimidating. It helps tremendously to invest in the available software specifically for CompuServe that provides automation of certain tasks, such as picking up e-mail, retrieving files and messages, etc., and for simply making navigation easier.