DS Errors are a special group of severe
Macintosh error codes, all with positive error codes (in the ranges 1 to 103, 20000 to 20002, and the catch-all 32767), which distinguishes them from the negative error-code I/O errors.
What does DS stand for?
Well, among Apple's technical support employees, DS informally stands for deep shit*. Really though, it actually stands for something less spicy: "default startup". Apple's HIG (Human Interface Guidelines) required that the error message string table (DSAT, default startup alert table) was to be one of the first resources loaded by the OS, in order to provide users with more useful, readable error information should any of them occur early in the boot process.
"Sorry, system error 27 has occured: (dsFSErr) the file system map has been trashed."
Because this string table was loaded so early, the Macintosh greeting ("Welcome to Macintosh") is in the table as error 40, dsGreeting. One of the most common errors is number 3, dsIllInstErr: "Illegal Instruction Error". In my opinion, the coolest one is number 31, dsNotThe1: "this is not the disk I wanted".
* My high school IT teacher related this to the class one day after he had phoned Apple about one of these errors.