1) Green Lantern was first created in 1940 and has since been reimagined multiple times with different characters taking on the mantle of Green Lantern.
2) The Green Lantern gets their powers from a power ring fueled by willpower that allows them to affect physical objects and fly. They are part of an intergalactic police force.
3) In the 1970s, Green Lantern comics addressed social issues of the time when he teamed up with Green Arrow, though this run did not last long. The character and titles have fluctuated in popularity over the decades.