What do intrusions and extrusions do?

Extrusive rocks are... extruded... onto the surface as the lava that we all think of. It cools fairly quickly, into rocks with fairly small crystals (might need a magnifying lens to see them). The exact chemistry of the parent magma will determine the type of rock. Oceanic volcanoes are generally basaltic (mafic or ultramafic), continental are generally granitic (felsic or intermediate). They're put down in "relatively thin" layers.

Intrusive rocks cool for thousands of years beneath the surface in huge plutons or batholyths. This allows them to grow very large crystals (easily seen by the unaided eye). These intrusive rocks are almost always granitic-type rock.