Wiki knows official stuff:
https://en.wikipedia.org/wiki/Sponge_and_dough
I just think of a sponge as the 'pre-dough'. Mostly only it ever comes up when I'm making two-day or longer breads or rolls or what-have-you. Especially when working with a starter, you know? So you've got your flour-water-yeast mix, and you scoop out a cup and add more flour-water and let it sit long enough for the yeast to multiply through it and then add your everything else to make dough out of.
That wiki page looks super specific and scientific and I have no clue about all those things. I just eyeball everything and adjust later.