What exactly is a QRH?
A QRH is a Quick Reference Handbook and is an approved (by some overseeing entity) document specific to an airplane and operator that contains checklists for just about anything the airplane can experience.  At my airline we had copies in the airplane and also in our company flight manual (CFM).

The entry point into a QRH checklist is one of a few things


Memory items
EICAS message
An abnormal event


For option 1, there are various checklists that must be memorized and performed at a moments notice with no reference to the checklist.  Some of these are things like trim runaways, engine fire/severe damage, emergency descent.  Once the memory part of the checklist is complete you refer to the printed version of the checklist in the QRH to verify and perform additional steps beyond the part you were required to memorize.

For the other options, any time anything abnormal happens or an EICAS message is presented, there are indices by procedure name and also by EICAS message into a specific procedure in the QRH to follow.  These procedures may reference other procedures.

For example, If I were to experience a dual engine failure, I would pick up my QRH and find the procedure "DUAL ENGINE FAILURE", this procedure says I should have the master warning light illuminated (and an aural triple chime) and I should see the EICAS message (in red) "ENG 1-2 OUT".  After verifying this presentation and verifying I am in the right procedure, I would perform:

Airspeed....................MINIMUM 260 KIAS
Altitude....................MAXIMUM FL250
Oxygen Mask (if required)...DON
APU.........................START
ENGINE AIRSTART procedure...PERFORM  (note, this is another QRH procedure)

CAUTION: IF THE APU HAS BEEN USED TO START THE ENGINES, 
         DO NOT ALTERNATE THE FADECs AFTER START

Note: If the APU is not available, only equipment supplied by the 
      Essential DC Bus 1 and 2 will be available.  Engine windmilling 
      should drive engine driven hydraulic pumps and supply hydraulic
      pressure.

IF neither engine can be restarted:
    DITCHING or FORCED LANDING procedure......PERFORM