Unlike when I got into comms, the internet is cheap and readily accessible. My first modem was a V21(300/300) / V23(1200/75) Miracle Technology WS4000, without the luxuries we now take for granted: Data compression, error correction, etc, etc.
Back when I started, in the early 80's, the order of the day for modem owners were BBS's, or Bulletin Board Systems. Usually, only a local telephone call away, thankfully. BBS's offered, and still do offer, all the things that the internet has, but generally speaking, it's all done by hobbyists for little or no financial reward. In fact, usually at great personal cost.
Of course, in the early 80's computers weren't as powerful as they are now. Hobbyists had machines like the Commodore 64, Sinclair Spectrum, Atari 800 for example. All of these machines used an 8-bit, rather than the current 32 or 64 bit processors, and none of them had multi-tasking capabilities. All this meant that one had to load the communication program, call the BBS, read any messages and reply to them whilst online. Once that had been done, you could then proceed to download any files of interest, and upload any that you thought others may be interested in, and at 300 baud, it could take a long time. Not very helpful on the pocket, all in all.
As time passed, and computer technology improved, and modems got faster, it became possible to call a BBS, download your messages, using a comms package, read and reply to them whilst offline, then upload your replies next time you called the BBS. Various systems were devised for this, but the most popular were QWK (pronounced quick, unsurprisingly) and BlueWave. Both systems allowed the packing and unpacking of messages to be up/downloaded, but the two systems are incompatible.
It was only a matter of time before people started to want to pass information between BBS's, and chat to users of other BBS's, but without calling the other boards. A few people in the USA came up with a system of allowing one BBS to call another BBS, and pass mail between the two of them; FidoNet was born.
Essentially, FidoNet offers similar functionality to that offered by the internet, but at a reduced speed. For example, mail that reaches Australia from the UK within minutes via the internet, can take weeks using FidoNet. Or rather, could; In the past, mail destined for Australia (for example) from the UK would be passed through a series of BBS's (called NODES) that telephoned other BBS's at certain times of day, usually in the very early hours of the morning. Since the period of time during which BBS's telephone each other is usually limited to one or two hours, and boards may be engaged, or offline for various other reasons, mail doesn't always get delivered every day. Furthermore, only a finite number of boards can be called in that given time, anyway. As a result, it can take anything up to a week, sometimes more for mail to reach it's destination. Nowadays, FidoNet communication between UK and Australia is somewhat faster. How? Because international bound messages get routed via BBS's that have internet "gateways". That is, a BBS that can convert the packed mail packets into a form suitable for transmission over the internet. Even so, reply times are measured in days, not hours.
If you are interested in finding out more about FidoNet, visit the official Fido website at Fidonet Home Page