The QRH has index or table of contents on where to find different abnormalities. They are usually divided into seriousness of of the situation either emergencies, abnormalities, advisories or status messages on the EICAS (engine indicating and crew alerting system) which would have different colour codings as well. Then they are indexed depending on the system that is in question for example power plant for engine problems, electrical, fire protection and so on. Sometimes different scenarios require the flight crew to refer to other pages in the QRH and all this while trying to fly the airplane and maintain situational awareness. This is primarily the one important reason why pilots can never be replaced by computer alone and why we still need at least two pilots for (most) commercial operations. QRHs can range from a few pages to a few hundred pages depending on the simplicity or complexity of an aircraft.
It makes the pilots life and workload that much easier because there is simply no way any pilot can memorize every possible scenario that can arise from a flight. The QRH tries to put all that into one little package...even then the randomness of situations still occur and pilots eventually will have to use their judgment if the QRH doesn't outline the problem.
