There are 48 shield volcanoes on Earth (active & inactive, excluding the extinct ones). Out of approximately 1,500 volcanoes in total on the planet, shield volcanoes represent about 3%.
Shield volcanoes are composed for the most part of basalt, which is fluid lava. Their name comes from their aspect as seen from the side, looking like a shield lying on the ground. This distinctive shape comes from the build-up of broad lava sheets which spread all around the magma chamber.
The most famous ones are in Hawaii, such as Kilauea and Mauna Loa, the largest volcano on Earth.