GEDCOM is a text file which uses various keywords and text data to describe a family tree. For example here's the start of the bit about myself:
GEDCOM line | Comment |
0 @I1@ INDI | I am individual number 1 in this tree. |
1 NAME Andrew /ALSTON/ | That's my full name. |
1 SEX M | I'm male. |
1 CHAN | The record for this individual was last changed... |
2 DATE 01 JUN 2018 | ...a couple of years ago. |
Because the file is in plain text, virtually any computer can be used to handle them.
The various record types are defined, but many implementations extend them. For example, the software I use defines various censuses using a keyword EVEN (short for EVENT) to describe arbitrary events.
Any genealogy software worth the name will allow you to import and export files in GEDCOM format. The current "standard" is many years old now. There are proposals for an update, but until the document becomes official, few software authors are going to spend effort on it.
Of course, not every brand understands those extensions. Ancestry can understand those EVEN types when importing a GEDCOM, display them properly, and will even re-export them later. I sometimes export a GEDCOM from Ancestry, run a script to change "Residence" events to the relevant "Census yyyy" events, then import it back, just because I like to see the censuses clearly.
Things get more awkward when it comes to describing the sources involved because various brands handle them differently. Ancestry converts my sources into "Unsourced Citation" types, but the result is at least accessible. Within Ancestry, a source consists of a pointer to somewhere in their databases rather than, say, a National Archives reference. If you create a GEDCOM from there, the relevant text line will contain a set of pointers into their data. Re-importing to Ancestry will re-link those same records (provided your account has access). Importing these things into the software I use throws away the pointer to the image, but does leave the text part so that I can locate it online. Some software can follow these pointers and actually download the image for you.
As I mentioned, pretty much any genealogy software will understand a GEDCOM from Ancestry. However a little tweaking might be needed to make it look "pretty" afterwards.
You can export and download a GEDCOM from Ancestry at any time - it is not destructive.