MODELLING CONCEPTS
Quads.
Any face with exactly four edges connected in sequence is a quad.
Quads are great. We strive to make all of our models out of only quads. The subdivision surface modifier works perfectly and predictably with quads.
Your surface continuity will be predictable and deformation will work well. UV unwrapping and Texturing become relatively simple tasks and your models will be easily understood by other artists and all software.
We understand the mathematics of the various poles and how they react to light when quads are used.
Quads are the gold standard in subdivision surface modelling.
Quads
Quads in 3d are considered to have flow which other arangements of vertices do not (triangles and n-gons do not have flow.)
In order for a face to have flow it needs to be made of exactly 4 edges arranged in a convex topology. The flow can enter any of the four edges which make up the face and will exit through the opposite edge.
This is not possible with triangles as there are only adjacent edges.
There are too many options as the exit edge when using N-gons.
Good subdivision surface modelling relies on the idea than a mesh will be made entirely from quads. If only it was that simple! Maintaining loops of quads over a surface can be tricky. Not always possible, but certainly the goal.
'flow' moving through a quad based mesh