Many sources divide Geometric Sans and Geometric Slab Serif fonts into two separate categories.
Geomeric sans are Metro, Kabel, Avenir, Erbar Grotesk, Avant Garde Gothic, Neuzeit Grotesk; squared like Bank Gothic, Microgramma, Eurostile. Geometric Slab were established at the same time (in the beginning of the XX century) as opposed to Slab: Beton, ITC Lubalin Graph, Memphis, Rockwell.
However, they have a lot in common and could be put in the same category. Compare these two typefaces: Futura (Sans) and Stymie (Slab). They both are based on simple geometric forms (just look at letters "o" and "a"): square, equilateral triangle, circle.
Minimalism, modern appearance and simplicity are the essential characteristics of Geometric fonts. Perhaps they have little "warm" and individuality but very functional and versatile. This certainly make them so popular, especially in using at web-design.