What is GEDCOM?
GEDCOM is a "standard" format devised by the Church of the Latter Day Saints to support GEnealogical Data COMmunication of family history records.
There is an excellent and detailed article on the history and use of GEDCOM on the Encyclopedia of Genealogy.
There is also a useful GEDCOM page in Wikipedia.
Family Historian stores all its information directly in a GEDCOM file. This means when sharing information with other users you can send the file directly to them. However, in practice most people use a variety of tools to prepare their data.
These include the Split Tree Helper tool, File > Import/Export > Export > GEDCOM File, external Utility ~ RJT Family Historian Converter and Utility ~ Res Privata - Gedcom Privatisation, plus Family Historian Plugins.
One thing which is worth remembering is that most other genealogy programs do not use GEDCOM as their standard format as Family Historian does, so when sending a file to people not using it, make sure you tell them they will need to use the Import or Merge option on their program to load the data into their program.
In addition to the standard TAG fields defined by GEDCOM, it allows custom fields to be added using an "_" on the front of the TAG.
Family Historian uses these to support additional functionality. See the GEDCOM Extension List.