A character's short description or sdesc is just what it sounds like -- a short description summarizing some key identifying traits of your character. A character's sdesc, rather than their name, is what other players see when they first encounter the character on screen. The sdesc is set during character creation, and cannot be changed afterwards except by special request.
Examples: a tow-headed gangly young man, a stout black-haired inspector, a compact woman with an intense dark gaze
To assign a name, nickname, or other alias to a character, the RECOGNIZE command is used. Once an alias is assigned this way, it will appear to you in place of the character's sdesc. Other players, however, will continue to see that character's sdesc or whatever alias they themselves have assigned, even if you target that character using your own alias for them.