Volcanic activity
Popular classification of volcanoes
A popular way of classifying magmatic volcanoes is by their frequency of eruption, with those that erupt regularly called active, those that have erupted in historical times but are now quiet called dormant or inactive, and those that have not erupted in historical times called extinct. However, these popular classifications—extinct in particular—are practically meaningless to scientists. They use classifications which refer to a particular volcano's formative and eruptive processes and resulting shapes, which was explained above.
Active
There is no consensus among volcanologists on how to define an "active" volcano. The lifespan of a volcano can vary from months to several million years, making such a distinction sometimes meaningless when compared to the lifespans of humans or even civilizations. For example, many of Earth's volcanoes have erupted dozens of times in the past few thousand years but are not currently showing signs of eruption. Given the long lifespan of such volcanoes, they are very active. By human lifespans, however, they are not.
Scientists usually consider a volcano to be erupting or likely to erupt if it is currently erupting, or showing signs of unrest such as unusual earthquake activity or significant new gas emissions. Most scientists consider a volcano active if it has erupted in the last 10,000 years (Holocene times) – the Smithsonian Global Volcanism Program uses this definition of active. There are about 1500 active volcanoes in the world – the majority along the Pacific Ring of Fire – and around 50 of these erupt each year.[8] An estimated 500 million people live near active volcanoes.
Historical time (or recorded history) is another timeframe for active. The Catalogue of the Active Volcanoes of the World, published by the International Association of Volcanology, uses this definition, by which there are more than 500 active volcanoes.However, the span of recorded history differs from region to region. In China and the Mediterranean, it reaches back nearly 3,000 years, but in the Pacific Northwest of the United States and Canada, it reaches back less than 300 years, and in Hawaii and New Zealand, only around 200 years.
As of 2013, the following are considered Earth's most active volcanoes:
Kīlauea, the famous Hawaiin volcano, has been in continuous, effusive eruption for thirty years, and has the longest-observed lava lake
Mount Etna and nearby Stromboli, two Mediterranean volcanoes in "almost continuous eruption" since antiquity
Mount Yasur, in Vanuatu, has been erupting "nearly continuously" for over 800 years
The longest currently ongoing (but not necessarily continuous) volcanic eruptive phases are:
Mount Yasur, 111 years
Mount Etna, 109 years
Stromboli, 108 years
Santa María, 101 years
Sangay, 94 years
Other very active volcanoes include:
Mount Nyiragongo and its neighbor, Nyamuragira, are Africa's most active volcanoes
Piton de la Fournaise, in Réunion, erupts frequently enough to be a tourist attraction
Erta Ale, in the Afar Triangle, has maintained a lava lake since at least 1906
Mount Erebus, in Antarctica, has maintained a lava lake since at least 1972
Mount Merapi
Ol Doinyo Lengai
Ambrym
Arenal Volcano
Pacaya
Klyuchevskaya Sopka
Sheveluch

Extinct
Extinct volcanoes are those that scientists consider unlikely to erupt again, because the volcano no longer has a magma supply. Examples of extinct volcanoes are many volcanoes on the Hawaiian – Emperor seamount chain in the Pacific Ocean, Hohentwiel, Shiprock and the Zuidwal volcano in the Netherlands. Edinburgh Castle in Scotland is famously located atop an extinct volcano. Otherwise, whether a volcano is truly extinct is often difficult to determine. Since "supervolcano" calderas can have eruptive lifespans sometimes measured in millions of years, a caldera that has not produced an eruption in tens of thousands of years is likely to be considered dormant instead of extinct. Some volcanologists refer to extinct volcanoes as inactive, though the term is now more commonly used for dormant volcanoes once thought to be extinct.

Dormant
It is difficult to distinguish an extinct volcano from a dormant (inactive) one. Volcanoes are often considered to be extinct if there are no written records of its activity. Nevertheless, volcanoes may remain dormant for a long period of time. For example, Yellowstone has a repose/recharge period of around 700,000 years, and Toba of around 380,000 years.Vesuvius was described by Roman writers as having been covered with gardens and vineyards before its eruption of AD 79, which destroyed the towns of Herculaneum and Pompeii. Before its catastrophic eruption of 1991, Pinatubo was an inconspicuous volcano, unknown to most people in the surrounding areas. Two other examples are the long-dormant Soufrière Hills volcano on the island of Montserrat, thought to be extinct before activity resumed in 1995 and Fourpeaked Mountain in Alaska, which, before its September 2006 eruption, had not erupted since before 8000 BC and had long been thought to be extinct.
Source: en.wikipedia.org