Basically it combines three existing modes into one box and saves space. Traditionally Hams send morse with a straight key , or a Vibroplex bug key:-
http://en.wikipedia.org/wiki/Vibroplex
That just plugs into the rig. To receive you get the morse out as a series of bleeps, which you write down.
For RTTY (Radio Teletype) you had a Teleprinter, in the US a Teletype 28 , in the UK a Creed 7 or 75 and in Europe perhaps a Siemens or other German make. You connect this to your rig using typically an ST-5 terminal unit which is a modem and teletype loop driver in a single box.
For slow Scan TV, which is really a form of FAX as its still pictures you had a SSTV unit that typically had a scope tube with a long persistence phosphor (P7) so the image doesn't fade too much while its being displayed. There is a wiki but its not very good...
http://en.wikipedia.org/wiki/Slow-scan_television
Any way instead of using a separate Morse Code Key, Slow Scan TV receiver and Teletype for RTTY which took up loads of space you have one box that can do all three. You wouldn't do all three together, of course.
Hope that explains things
Dave.