Low-poly modelling

OK, I’m kind of new to game modelling, but the models I’m looking at for examples use triangles. Is this because graphics cards can’t render quads? or is there another reason?

Probably because when you use a quad it’s actualy composed of two triangles. When modelling in low-poly you will want to see how the triangles will be rendered by the GPU so you can flip them around. A quad with one triangle orientation will look all different when the triangles are flipped. With low-poly it does show. It’s rather difficult to explain but you’d know if you had done low-poly characters for example. Maybe I should try to make a pic to explain this better :-?
A fast example. The same geometry. The one in the left is what Blender did when I moved the top faces up. On the right side is the same geometry but fixed with a triangle flip (ctrl+FKEY) The wire on the right has the two edges missing because it’s not the real wire but rather rendered with a wire material. In the 3d viewports they show properly.