It's a primarily Unix-based typesetting package, older and less sophisticated than LaTeX. Nowadays its probably used most for Unix manual pages, since it's the native format, but I'm sure I'm not alone in still using it for other purposes. It enables you to define your own macros so once you're familar with the system you can be very quick, e.g. I have one macro that starts a letter complete with my address, recipients address (in the correct place for a window envelope - takes experimentation in a word processor) and date automatically. This might not seem special but to get it all I need do is type ".LH" at the beginning of a line. Similarly I have a macro to insert a signature block at the end of the letter - it will even insert my signature if I tell it. After all, I'm far too important to be wasting time signing my own letters. ;-)