The data format of JabRef is BibTeX. In addtion JabRef also supports biblatex.
JabRef is is a program for working with BibTeX and biblatex libraries. JabRef program uses no separate internal file format but directly works with BibTeX and biblatex. That means, your BibTeX/biblatex file is kept as is when opening in JabRef and saving again: You normally load and save your libraries directly in the BibTeX/biblatex.bib format. In addition, you can also import and export bibliography libraries in a number of other formats into JabRef.
The library mode can be changed in the library properties.
More information on BibTeX is available on our information page on BibTeX fields.