The difference is that the intrusive rock, has a lager crystal texture; due to the slow cooling. While the extrusive rock, has a much faster cooling; therefor has less of the crystal texture. The intrusive rock also crystalizes bellow the surface; while extrusive rocks erupt onto the surface. I hope this helped. I told you, chemistry is not my best subject. Just so you know, I did not copy the answer off of google, this is just my understanding of the rocks.