What is the difference between BibTex and BibLaTeX database in the context of JabRef?
BibLaTeX is the succesor format of BibTeX, it is the more modern format, which enhances bibtex.
JabRef supports both formats. In the context of JabRef the main difference is just the different entry types and corresponding fields.
Despite that, JabRef supports Cleanup operations to convert from BibTex to Biblatex, e.g move the content from journal to journaltitle.
Thank you very much for your explanation!
It sounds like I should use BibLaTeX when creating a new database.
But why then is the “default bibliography mode” set to “Bibtex”?
Because, despite many years, BibTeX is still the de-facto standard that most users know. Moreover, most academic publishers (that support LaTeX) do not support BibLaTeX and require references in either BibTeX format or something compiled thereof.
The step from BibLaTeX to BibTeX or something else you can publish is not mind-bogglingly difficult, but at some point every additional step you have to take becomes annoying.
Note that this is not meant as an argument in favor of either format. I am agnostic towards these formats and you should probably use what you are most comfortable with.
A good comparison of BibTeX and BibLaTeX can be found on stackexchange and a related question about journal publications using biblatex.