What is meant by intrusive or extrusive rocks?

Intrusive or extrusive are terms given to igneous rocks which describe where the rock cooled and formed, since this plays an important role in the cooling time of the rock and therefore grain size of the rock. Intrusive rocks are those that cool from magma before it reaches the surface of the earth (they form inside the earth, hence the term intrusive). Extrusive rocks form by erupting or "extruding" at the surface and cooling. Intrusive rocks take a long time to cool because the surrounding rocks act as a kind of blanket and keep it warm longer, so crystals are generally large, where extrusive rocks cool quickly being exposed to air, wind, and rain, so crystals are generally small.