Quads, tris, videogames

FullTomates

One of the few things I know of 3d modeling is that the geometry needs to be all tris or all quads. but im seeing some models with mixed polys.
Can someone explain me this? Im really confused.

cryrid

One of the few things I know of 3d modeling is that the geometry needs to be all tris or all quads.... 
...Can someone explain me this? Im really confused.

It's really a simple explanation... I'm afraid that someone has completely misled you. Geometry does not need to be one or the other. 

kanga

When you import a model into an engine all quads are converted to tris. when a quad is warped the engine will convert it but the conversion might not be edges in the direction you want, or it might not shade properly. So any warped polys should be divided into tris in the direction that helps baking and also shading in the engine. There are also times when tris are handy, by reducing geometry where it isnt needed, but the general rule is to model in quads where you can and use tris when needed. Also changing a mesh is easier if the polys are quads, so if you need to make repairs or alterations to the model quads are considered more efficient.