GEDCOM is, as stated above, an acronym. It is a standards based file that most genealogy programs can produce (export) that will allow you (the user) to transfer genealogy data from one genealogy program to another (import).
Unfortunatelly, no mainstream genalogy program follows the rules (standards) as set forth in the standards document. So the first rule of GEDCOM is that almost all GEDCOMs exported from one program will not transfer correctly to another program. HOWEVER, do not be discourage. Most people never (or only once) transfer data from one program to another program. After a little while they are "locked" into using the program they select and never change programs. This is both good and bad.
If you are starting by copying the work of other people and they used a program that can produce a GEDCOM, you will be starting with their GEDCOM as they export it from the program they use, then you will import the GEDCOM file into your genealogy program.
If you purchase a program other than the program they used most likely you will loose some data. Some of it will be of little importance, some may be very important.
Don't feel you must purchase the same program they used, it may not be very good or even available for purcahse today. It is better to purchase a program that does what you want and produces the reports, books, websites you want/need. The lossed data can usually be reentered.
FTM2012, does not need a GEDCOM to transfer information form Ancestry.com. It can do it without creating a GEDCOM.