Scientists classify volcanic eruptions based on the violence of the eruption and the type of materials that spew out of it. There are four basic types:

Hawaiian Eruptions Named for the volcanoes in Hawaii, these are the least explosive type of eruptions. Very fluid lava is erupted, usually in the form of lava flows and lava fountains.
Strombolian Eruptions These are named after the Stromboli volcano in Italy. Strombolian eruptions typically involve many small explosions that occur in short, regular bursts. These bursts are caused by the sudden release of bubbles of volcanic gas as the magma rises to the surface.
Vulcanian Eruptions Named for Vulcano, a volcanic island off the coast of Italy, these are relatively small eruptions that eject rock fragments, bubble-poor bombs of fresh magma, and abundant ash. Vulcanian eruptions, like all volcanic eruptions, are caused by buildup and then sudden release of volcanic gas pressure.
Plinian Eruptions These eruptions get their name from Pliny the Younger, who described the 79 A.D. eruption of Vesuvius, near Naples, Italy. These extremely explosive eruptions form eruptive plumes that reach over 11 kilometers high. The ash fall from these plumes can cover areas hundreds of kilometers downwind of the volcano.

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Photos: (1,2,4) USGS; (3) Andy Lockhart. Source: USGS Photoglossary of Volcanic Terms