A modem (modulator/demodulator) is used to transmit digital information via analog signals. In retro-computing, this usually means sound waves over telephone lines.

Appropriate topics pertain to aspects of modems particular to retro-computing devices. For example, while 300 baud signaling isn't popular anymore, it's not inherently retro, as modern devices typically support it (along with many other protocols). Features tied to a certain era, like US Robotics' HST, are suitably retro.