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 (fh) stores its information directly in a GEDCOM file. When sharing information with others, sometimes you can send this file directly to them, but from fh V6.0 it uses Unicode UTF-16 encoding with a BOM, which may not be recognised by other programs. However, in practice most people use a variety of tools to prepare their data.
These tools include the Split Tree Helper, Import/Export > Export > GEDCOM File, Utility ~ RJT Family Historian Converter, Utility ~ Res Privata - Gedcom Privatisation, and Family Historian Plugins such as Export Gedcom File.
One thing worth remembering is that most other genealogy programs do not use GEDCOM as their standard format in the way fh does, so when sending a file to people not using fh, make sure you tell them they will need to use the Import or Merge option to load the GEDCOM data into their program.
The standard fields defined by GEDCOM have Tag names, such as SEX, DATE, NOTE, or NAME, that are followed by their data value.
GEDCOM allows custom user-defined fields with an underscore (_) prefix on the field Tag names, such as _ROOT, _EMAIL, or _TYPE, to distinguish them from standard Tag names.
fh uses these to support additional functionality. See the GEDCOM Extension List.
Some genealogy products add custom fields without an underscore (_) prefix and are thus invalid GEDCOM. These and any other unrecognised field Tag names imported to fh are treated as Uncategorised Data Fields (UDF).