Not many years ago there was no '911' system. There were separate 7-digit phone numbers for each service, each with it's own dispatch. A separate number for police, fire, and ambulance had to be used. There was little in the way of a standard protocol for call-takers to use in order to gather accurate information, and information gathered was rarely shared between services.
This was highly inefficient, as most moderate-to-severe incidents require 2 or 3 of the services to respond (think motor-vehicle crashes; police to secure the scene, control traffic etc, fire to operate rescue equipment/put out fires, and ems to fix people. A better system that has begun to emerge is combined tri-service centres which dispatch all three from the same location. Information travels as fast as a click of a mouse - and everyone gets up-to-date info much faster. Most dispatch centres these days have moved to bi/tri-service, with police sometimes left to a specific separate centre for various reasons such as complexity and security.
With technology ever-improving and tele-com systems upgrading, most 911 centres have moved to enhanced-911 (e-911). This means that when you call, your phone number, and a database entry of your address from the phone company are delivered straight to the dispatcher's computer. This is referred in the industry as Automatic Number Information/Automatic Location Information (ANI/ALI). ANI/ALI Information for the most part is NOT delivered with cel-phones.
The next upgrade being deployed is the ability for the dispatcher to retrieve your GPS location direct from the embedded GPS in a cel-phone. Some areas already have this feature, MOST do NOT. It has been argued by opponents of the system that it invades your privacy - even though it is an option to enable or disable in your phone - it could arguably be abused by police to find suspects etc. Currently where the system is not enabled, the dispatcher is presented with the location of the cel phone tower you're using, and the angle (Azimuth) from the tower to your phone.
See the wikipedia article: e911
for more information.