Magma is melted rock stored in the Earth’s crust while lava is magma that reaches the surface in volcanoes or other vents.
Magma is a generic term for molten rock found beneath the crust while lava is the term for magma that has broken though to the surface. In terms of composition magma and lavas can be either felsic (quartz and feldspars mostly) or mafic (hornblende pyroxene micas). Feslic volcanic lavas (like Mt. St. Helen’s in the U.S.) tend to be very explosive while mafic flows as in Hawaii tend to be slow moving lavas.