Why generic names like "Sans" & "Serif"?

[Jorge Fábregas]

Hello everyone,

I've been using Linux for a couple of years but it hasn't been until recently 
that I've been paying attention to the font ecosystem so please bare with 
me :)

I'm wondering why on KDE & Gnome (and on apps like Firefox) whenever there's a 
widget to choose fonts there is always these generic names: Sans & Serif.  
With this I don't know which sans-serif font I'm using...or which serif font 
I'm using.  For example, when I go to the Firefox preferences...for 
sans-serif I have "sans-serif" and I'm like "Ok....but WHICH ONE? Is it 
Liberation Sans? Dejavu Sans?".  This is the confusion.

I'm guessing now: is it like this so you can define system-wide (at fontconfig 
level) the typeface you want for sans-serif family? and the serif family?  
So, for example, imagine I want to use in all system-apps the "Liberation 
Sans" font for all sans-serif requests...I'll then create a rule on my 
~/.fonts.conf for that and I wouldn't need to go to each app (or desktop 
environment) to specify that I want "Liberation Sans" for sans-serif?  I just 
need to make sure that I have "Sans" in all my preferences and I can 
double-check that by "fc-match Sans" right?

Please help me understand this :)

Thanks in advance,
Jorge