Low-poly is a design that was used originally in computer graphics to make 3D models for video gaming. Recently it has become a 2D trend used by artists and illustrators to create abstract pictures. The gaming models use more polygons and become more detailed than the 2D art but both can be very intricate.
The size and shape of the polygons help define the details of the picture. Smaller shapes are used for the more detailed areas and larger shapes for the less defined areas. Artists can also create focus on a certain area by using different shape sizes, exaggerating important features and adding more detail to that focusing point. Different colors can be added for shading and highlights and to show movement in a picture. Texture can be used for movement as well and to add depth.
Below are some examples of low-poly art. You can see how by varying the sizes, colors and textures of the polygons, different styles are created by the artists.