Lynn, the HST is the High Sierra Trail, which crosses the Sierra east-west. As opposed to the SHR, the Sierra High Route, which mirrors the John Muir Trail (JMT) north-south, only at higher elevations.
I believe most of the acronyms you'll see on this site and the WPS (Whitney Portal Store) are abbreviated landmarks along the MWMT (Mt. Whitney Main Trail, or MMWT for Main Mt. Whitney Trail). Like TC for Trail Camp (or sometimes Trail Crest - use context), LPL for Lone Pine Lake, OC for Outpost Camp, TM for Trailside Meadow, ML for Mirror Lake, CL for Consultation Lake, BHP or BP for Big Horn Park, JMT for the John Muir Trail junction.
Then there's the MR (Mountaineer's Route). NF for North Fork of Lone Pine Creek (LPC), EL for Ebersbacher Ledges, LBSL (Lower Boy Scout Lake), UBSL (Upper Boy Scout Lake), IL or IBL (Iceberg Lake), F400 (Final 400).
You'll also sometimes see other nearby trailheads along the eastern Sierra abbreviated: HM for Horseshoe Meadow, OV for Onion Valley, MF for Mosquito Flats.
And HAPE is High Altitude Pulmonary Edema (fluid in the lungs due to altitude). HACE is High Altitude Cerebral Edema (fluid/swelling of the brain for the same reason).
But that's only IMHO (in my humble opinion) and YMMV (your mileage may vary) . . .
