GREEN LANTERN debuted in Showcase #22 (October 1959, DC house ad shown above). The character was a modernized or "Silver Age" version of a Golden Age hero originally created by Bill Finger and Martin Nodell. The original GL mainly fought street crime, and tackled the occasional monster.
The new GL was an intergalactic policeman who fought evil with a super-scientific ring energized by his "will power" and capable of transforming thoughts into reality. He battled outlandish super-villains such as the Invisible Destroyer, shown below in a DC house ad for Showcase #23 (November 1959):