There usually had just the one line so users were limited to a certain number of minutes a day before they were no longer permitted to sign on. Sometimes a shareware game or new "Demo" would make to to a BBS and it might take you a week or two to download the entire thing because of time limits.
Some systems gave you extra time if you uploaded new content to encourage sharing. It was kind of a neat system, and I miss how small it was.
Just Google for "BBS" i.e. bulletin board system.
They worked over phone lines. Normally you would know about one BBS from a computer magazine and then that BBS would have a megalist of other ones on it.
Sometimes the ads were the payload: https://en.wikipedia.org/wiki/ANSI_art#Scene
and I recognized QSD immediately when I saw it in your post.
I found my way into some x.25-to-internet gateways in the early 90's. Most corporate networks were very insecure then. Some of those gateways had no authentication: you'd connect in over x25, then telnet out. The local colleges were also pretty bad.
Also, from the page's source code:
// UUDDLRLRBA! var kkeys = [], konami = "38,38,40,40,37,39,37,39,66,65";
:)