Polygon shapes.
 

In tutorial on cartoon dog, he said ,

"try not to have triangles, and definately stay away from five sided"

I see that when i have non-quads, changing to sub-d makes it kinda funkey looking..

But what is actual problem caused by having non-quads? does it eat up a lot of memory? cause weirdness in animating?

what is the reason that i should not have five sided polys or triangle polys?

Thank you guys, I really really think this website is the most helpful place i have found.