From The RadioReference Wiki
Simulcast refers to the process of transmitting the same signal from different tower locations over the same frequency at the same time.
For public safety communications, this typically means multiple towers: configured to transmit the exact same communications, on the exact same frequencies, at precisely the same time. This can result in better coverage of a wide-area or a dense area with higher populous and/or buildings. Such simulcast systems must use some form of very precise timing: to synchronize, their, multiple transmitting towers, or interference can result, from mis-timed (asynchronous) signals, destructively mixing, in a receiver i.e. scanner or even a subscriber's radio on that system.
Simulcast is not analogous to multiple-site wide-area-networked trunking systems, that use different frequencies, at each site; however, simulcast "cells" (several sites operating as one in a simulcast configuration) can be implemented in such systems.
Example: Washington County (example only) Has 4 Sites: north, east, west, south; each is listed as a "Simulcast Site" in the RRDB. These Sites cover their intended area with frequencies that differ amoungst each site, but share all "Common" Talkgroups. Generally each site carries its own traffic on the individual Sites, without a Dispatcher's "Console" interference, in extreme circumstances at the Dispatcher's Console a key press can enable all 4 Sites to "All talk", cross-linking them together; now some may argue that this is the only time the Washington County is truly "Simulcast", but henceforth it's just "A System" with 4 independent Sites, aka a Multi-Site System. These 4 Sites allow 4x the amount of traffic to be on the Washington County Public Safety System at any given time at the peril of Public Safety people not hearing what could be happening a few miles away on one of the other Sites.
North actually has 4 Tower locations (1 Simulcast Site)
East actually has 6 Tower locations (1 Simulcast Site)
West has 3 Tower Locations (1 Simulcast Site)
South has 5 Tower Locations (1 Simulcast Site)