Geometric illustrations are based on raw, basic geometric shapes--squares, circles, triangles, etc., but can include intricate layers (and layer blending), and be far richer in detail than you'd expect from such simple, primitive forms. They can be literal, abstract, even three-dimensional, and combined they can essentially generate any type of illustration you're looking for!
Not only are they a fantastic way to add more depth and layering to branding elements, they're also excellent as a visual counter-balance/contrast against organic and naturalistic styles of illustration.
Below are example of some of my favorite Geometrics inspiration taken from Dribbble, along with links to view more works from each artist:
Alina Starovoitova - Geometry Magic
Anthony Gribben - Poster 54 - Circular
Bram Vanhaeren - Kleurvorm 029
Anatolii Babii - Storage Objects Concept Illustration
Anthony Gribben - 3D Geometric Digital Poster Art
Toma Varsano - Somewhere In The Universe
Ray Dak Lam - Shape Study: 030
Ganna Sereda (Anna_leni) - Memphis Seamless Pattern
Nathaniel Navratil - Type Exploration