Grotesque fonts

Today while reading this page: Roboto: Google’s signature font is now open source, I saw this comment, in response to someone asking what is so special about this font, Roboto.

I don't know specifics about this particular typeface (and I haven't been "into fonts" for a while), but Roboto is pretty much another typeface with some Grotesk (e.g. Helvetica) lineage.

Who would name fonts grotesque? My knowledge about fonts begin and end in discerning them as serif and sans serif. Wikiepedia says :

Akzidenz-Grotesk is a grotesque (early sans-serif) typeface originally released by the Berthold Type Foundry in 1896 under the name Accidenz-Grotesk. One of the first sans serif typefaces to be widely used, its design influenced many later neo-grotesque typefaces after 1950.

Grotesque is a subcategory of sans serif fonts (they are grotesque, neo grotesque, humanist and geometric). It is not clear for me what is the defining characteristic of a grotesque font, even after reading those description; I can't identify a font by looking at it as grotesque or humanist, these categorisations more or less appears arbitrary to my untrained eye.