The Hardrock, Rockhopper and Stumpjumpers were the meat and potatoes of Specialized's offerings in the '90's.
Hardrocks were offered as entry level mountain bikes, typically with low end components, cro-mo frames. Serviceable, but generally nothing special. Aluminum alloy frames were added to the line in 1997. I had a '94 with SR Duotrack 7005 suspension fork. My wife has a '97 AX with alloy frame that was in service until this past year, thru many upgrades. Relatively upright and relaxed geometry for the non-racer.
Rockhoppers were the next up in the line, a mid-range bike with a midrange component mix. Lighter cro-mo frames, and later alloy frames. Rumour had it that sometimes they were the previous year's Stumpjumper frames, but I'd put little stock in that. The geometry was closer to race-like, more agressive than the Hardrocks. Weekend warrior bikes for wannabe and beginner racers.
Stumpjumpers were the mainstream flagship of the line - XC race bikes in trim levels from STX to full XTR. Either very nice butted cro-mo frames or the M2 metal matrix composite frames (special aluminum alloy with ceramic inclusions). Even higher end Stumpjumpers, with butted M2 frames were sold under the S-Works name, hardcore racing bikes. There were occasionally special rare production things like the Stumpjumper Epic carbon lugged frames to drool over too.
That's the short version.
J