When a volcano is active there is magma or lava moving around getting ready to make an explosion. When a volcano is inactive there is either rocks and bits of other stuff melting in the main vent making magma getting ready to become active or they are not doing anything, just sitting there like a mountain. The four main types of volcanoes are shield volcanoes, composite volcanoes, also called stratovolcanoes, cinder cones, and lava domes.
Composite volcanoes are the most abundant, and an example is Mount Fuji in Japan.
These volcanoes are formed from layers of ash. In contrast, shield volcanoes are gently sloping volcanic mountains built almost fully of lava flows. They were named on account of their shape , which is reminiscent of an ancient warrior's shield. These volcanoes are typically much larger then composite volcanoes. Lava domes and Cinder cones are much smaller than shield or composite volcanoes.