Great question! A volcano is classified, in part, according to what kind of eruptions it has. There are three types of volcanoes, the cinder cone, the shield, and the composite. The cinder cone has violently explosive eruptions, while the shield volcano has quiet eruptions, more like magma oozing onto the Earth's surface. The composite volcano, as the name suggests, is a mixture of both explosive and quiet eruptions.
The ingredient in the magma that determines the type of eruption is the amount of silica content. Silica is a substance that tends to thicken the magma, causing it to clog the vents from which the magma erupts. Pressure builds because of trapped gases, and when the pressure exceeds the holding strength of the clog, you get an explosion of magma and hot gases. Low silica content produces thin, runny magma, typical of the shield volcano. Magma of this type seeps from the vent and runs across the surface of the Earth, much akin to pancake syrup flowing across a pancake surface.