A computer file standard, created by the Church of Jesus Christ of Latter-Day Saints (the Mormons). It stands for Genealogical Data Communication. All genealogy programs on the computer can read and write to this standard (as well as their own standard if not). There are many genealogy programs available, as well as on-line sites where data can be entered (ancestry.com, familysearch.com, etc.) and some of these allow you to upload a GEDCOM.
In addition, reports and charts can be made that are easier for people to read and share.