Volcanoes are not really classified in the way that tornadoes hurricanes and earthquakes are. They are either active or dormant. And if they are active, how dangerous they are.

An active volcano is a volcano that has either erupted recently, or has had a history of immediate explosions, or that has smoke over it.

A dormant volcano on the other hand is a volcano that has not erupted in a long time, and shows not signs of erupting. However, even a dormant volcano can erupt, as shown in the past.

More information about how/why volcanoes erupt can be found [here]

More information about what a volcano is can be found [here]

Information about major past volcanoes can be found [here]

Volcano links can be found [here]

More volcano pictures can be found [here]