Back in the day, before the World Wide Web came into vogue, there were things called Bulletin Board Systems (BBS for short). You would turn your computer on, open telecom, and dial up your favorite BBS. If that line was busy, you just moved down the list you had collected. A BBS was a lot like a large website, but on a much smaller scale...if that makes sense. Each BBS had standard features: A file section, e-mail, message boards, Doors (online games). If they had 2 or more modems and phone lines they might have had a chat feature as well.
Unlike all the fancy graphics and fonts you see on the internet, almost everything looked like this:
[F]ile Tranfers [E]mail [M]essages [D]oors
Once the internet took hold, they kind of faded out. Now, they're still around, but are generally accessed through Telnet rather than direct dial with a modem.
If you're bored between classes, maybe you should pull up Telnet and open a connection to gameland.darktech.org and check out the most popular door game aroud: Legend Of the Red Dragon.