Grotesque fonts, also known as Grotesk font or sans-serif grotesque fonts, are a category of typefaces characterized by their simple, straightforward, and sans-serif design. From a broad view, grotesque is used as a synonym for sans serif fonts in general. More specifically, grotesque refers to the set of sans serif fonts produced around 1815. Sans serifs from this time period and the following few decades are called grotesque and also sometimes Grotesk or Gothic.
You can recognize a grotesque in a few ways. Grotesques have a slightly crude appearance and a lot of visual character. These early sans serifs had less polish and more quirkiness than their more clinical and sleek contemporary counterparts, the Neo-Grotesques (think Helvetica). Grotesques are usually geometric in design with simple letterforms and fairly even stroke weights and they are also often bolder and can be used as display type.