A GEDCOM (GEnealogical Data COMmunications) is a text file designed to enable transfer of family tree information between different genealogy software programs. There is an agreed standard for these files, currently version 5.5. It is not 100% fool proof but pretty close. Some software programs vary slightly in exporting their data and importing another program's data, but these are usually minor issues. You can create "tags" for your own personal use so long as you follow the GEDCOM standard, but the receiving program needs to have the flexibility to receive non-standard tags.

The great thing about it is the ease with which you can share information between programs. One of the reasons I chose TNG for our web site is that I can keep the information up to date simple by uploading a full or partial GEDCOM file from my computer program to the web-site program. Update GEDCOMs are usually less than 100kb in size. Even the large one is only 1.8mb.

Read more about it at:

• Wikipedia: GEDCOM - general introduction
• GEDCOM 101 - explains the GEDCOM file's structure
• Cyndi's List - a large collection of links