From The RadioReference Wiki

"Talkgroup" is a generic term that refers to virtual radio channels created for/by a Trunked Radio Systems(TRS). While not actually a group, a Talkgroup ID (TGID) is the digitally assigned (DEC or HEX(adecimal)) virtual channel; in non-scanner speak it could be called a digitally assigned "user-group" channel, or a signal agency's digital channel identification number. Inherently in most "System's" operations, a group of people (users/subscribers) utilize one single TGID for voice communications, hence Talkgroup.

Additional Info and Examples:

  • In some scanners when adding/creating the digital virtual channel, it's labeled as a Channel, a TGID, or a Talkgroup
    • Note: some confusion may come from Uniden's Dynamic Memory Architecture DMA where you have the term "Group" essentially where TGIDs' ares kept.
    • Many, will notice, in a lot of TRSs, across the Radio Reference Database (RRDB), that TGID numbered by a "localized system admin" for a "jurisdiction's TRS" do generally, "group", like agencies with some sort of a pre-fix number-grouping-system for the TGIDS.
      • Example: Fire 16xxx, Police 17xxx, State Police 18xxx.
  • In most TRS there could be hundreds of "user-groups" / Talkgroups
    • Example: Highway Supervisors, Janitors at the Courthouse, Police Tactical Team, Trash man, Animal Catcher, Bus Drive, Fire Marshall, and Coroner so on and and so forth, all have their very own TGID;
      • each are allowed an empty frequency on a TRS to carry communication when needed, for the each individual subscriber.
      • some TGIDs may have "Priority" over other TGID's, leading to some subscribers on a specific TGID to constantly say they can't "get out", (basically when all the "System frequencies" are being used by "higher priority user-groups" (TGIDs)).

See the Trunking Basics for more explanation.